Cleanup Nullable tests
Cleanup Nullable tests

- Take advantage of xunit
- Refactor long tests

Contributes to #6286

nchikanov/corefx,dhoehna/corefx,dsplaisted/corefx,gkhanna79/corefx,tijoytom/corefx,marksmeltzer/corefx,stephenmichaelf/corefx,Priya91/corefx-1,krk/corefx,Priya91/corefx-1,billwert/corefx,JosephTremoulet/corefx,Petermarcu/corefx,zhenlan/corefx,ericstj/corefx,wtgodbe/corefx,cydhaselton/corefx,Petermarcu/corefx,tstringer/corefx,ravimeda/corefx,twsouthwick/corefx,SGuyGe/corefx,dotnet-bot/corefx,cartermp/corefx,nchikanov/corefx,alexperovich/corefx,wtgodbe/corefx,shahid-pk/corefx,Petermarcu/corefx,Ermiar/corefx,rjxby/corefx,Jiayili1/corefx,twsouthwick/corefx,JosephTremoulet/corefx,rahku/corefx,shmao/corefx,nchikanov/corefx,the-dwyer/corefx,shimingsg/corefx,the-dwyer/corefx,alexperovich/corefx,stone-li/corefx,rubo/corefx,ericstj/corefx,the-dwyer/corefx,tstringer/corefx,ravimeda/corefx,stone-li/corefx,weltkante/corefx,tstringer/corefx,dhoehna/corefx,ptoonen/corefx,alexperovich/corefx,tstringer/corefx,krytarowski/corefx,the-dwyer/corefx,nbarbettini/corefx,dhoehna/corefx,rahku/corefx,shmao/corefx,ViktorHofer/corefx,MaggieTsang/corefx,shimingsg/corefx,nbarbettini/corefx,ericstj/corefx,krytarowski/corefx,dsplaisted/corefx,stephenmichaelf/corefx,manu-silicon/corefx,axelheer/corefx,cartermp/corefx,ptoonen/corefx,weltkante/corefx,ravimeda/corefx,manu-silicon/corefx,jlin177/corefx,Petermarcu/corefx,rahku/corefx,nbarbettini/corefx,alphonsekurian/corefx,DnlHarvey/corefx,SGuyGe/corefx,stephenmichaelf/corefx,shahid-pk/corefx,rubo/corefx,shahid-pk/corefx,tstringer/corefx,Priya91/corefx-1,cydhaselton/corefx,MaggieTsang/corefx,dhoehna/corefx,twsouthwick/corefx,cartermp/corefx,tijoytom/corefx,krytarowski/corefx,dotnet-bot/corefx,iamjasonp/corefx,fgreinacher/corefx,nbarbettini/corefx,Ermiar/corefx,rubo/corefx,lggomez/corefx,DnlHarvey/corefx,seanshpark/corefx,stone-li/corefx,jhendrixMSFT/corefx,seanshpark/corefx,twsouthwick/corefx,weltkante/corefx,cydhaselton/corefx,khdang/corefx,mazong1123/corefx,shmao/corefx,jhendrixMSFT/corefx,gkhanna79/corefx,shmao/corefx,ellismg/corefx,ViktorHofer/corefx,manu-silicon/corefx,dotnet-bot/corefx,zhenlan/corefx,shahid-pk/corefx,shahid-pk/corefx,iamjasonp/corefx,JosephTremoulet/corefx,DnlHarvey/corefx,shimingsg/corefx,elijah6/corefx,yizhang82/corefx,shmao/corefx,manu-silicon/corefx,MaggieTsang/corefx,shmao/corefx,iamjasonp/corefx,alexperovich/corefx,ravimeda/corefx,parjong/corefx,wtgodbe/corefx,Priya91/corefx-1,stephenmichaelf/corefx,lggomez/corefx,nbarbettini/corefx,richlander/corefx,elijah6/corefx,jhendrixMSFT/corefx,seanshpark/corefx,jlin177/corefx,krk/corefx,khdang/corefx,the-dwyer/corefx,alexperovich/corefx,ellismg/corefx,lggomez/corefx,weltkante/corefx,yizhang82/corefx,yizhang82/corefx,ellismg/corefx,stephenmichaelf/corefx,elijah6/corefx,krytarowski/corefx,DnlHarvey/corefx,ViktorHofer/corefx,Petermarcu/corefx,mazong1123/corefx,parjong/corefx,iamjasonp/corefx,axelheer/corefx,ViktorHofer/corefx,cydhaselton/corefx,nchikanov/corefx,Jiayili1/corefx,billwert/corefx,fgreinacher/corefx,twsouthwick/corefx,mazong1123/corefx,gkhanna79/corefx,cartermp/corefx,shmao/corefx,alphonsekurian/corefx,jhendrixMSFT/corefx,YoupHulsebos/corefx,krk/corefx,adamralph/corefx,axelheer/corefx,marksmeltzer/corefx,alphonsekurian/corefx,yizhang82/corefx,richlander/corefx,dotnet-bot/corefx,adamralph/corefx,stone-li/corefx,ericstj/corefx,khdang/corefx,krytarowski/corefx,elijah6/corefx,marksmeltzer/corefx,stone-li/corefx,weltkante/corefx,jlin177/corefx,cartermp/corefx,elijah6/corefx,rubo/corefx,Jiayili1/corefx,wtgodbe/corefx,rjxby/corefx,DnlHarvey/corefx,lggomez/corefx,richlander/corefx,lggomez/corefx,zhenlan/corefx,yizhang82/corefx,mmitche/corefx,alphonsekurian/corefx,rjxby/corefx,DnlHarvey/corefx,SGuyGe/corefx,yizhang82/corefx,parjong/corefx,BrennanConroy/corefx,rjxby/corefx,zhenlan/corefx,zhenlan/corefx,alphonsekurian/corefx,krk/corefx,weltkante/corefx,wtgodbe/corefx,mazong1123/corefx,shahid-pk/corefx,ptoonen/corefx,marksmeltzer/corefx,tijoytom/corefx,MaggieTsang/corefx,Chrisboh/corefx,Jiayili1/corefx,Chrisboh/corefx,Chrisboh/corefx,stone-li/corefx,manu-silicon/corefx,JosephTremoulet/corefx,YoupHulsebos/corefx,the-dwyer/corefx,jhendrixMSFT/corefx,ptoonen/corefx,dhoehna/corefx,jhendrixMSFT/corefx,rahku/corefx,ellismg/corefx,rjxby/corefx,DnlHarvey/corefx,parjong/corefx,dotnet-bot/corefx,YoupHulsebos/corefx,billwert/corefx,cartermp/corefx,SGuyGe/corefx,ravimeda/corefx,parjong/corefx,dotnet-bot/corefx,stephenmichaelf/corefx,parjong/corefx,BrennanConroy/corefx,richlander/corefx,jhendrixMSFT/corefx,tijoytom/corefx,Chrisboh/corefx,twsouthwick/corefx,parjong/corefx,mazong1123/corefx,gkhanna79/corefx,MaggieTsang/corefx,alexperovich/corefx,marksmeltzer/corefx,shimingsg/corefx,fgreinacher/corefx,manu-silicon/corefx,jlin177/corefx,gkhanna79/corefx,shimingsg/corefx,Ermiar/corefx,Chrisboh/corefx,JosephTremoulet/corefx,seanshpark/corefx,ellismg/corefx,richlander/corefx,YoupHulsebos/corefx,wtgodbe/corefx,iamjasonp/corefx,MaggieTsang/corefx,richlander/corefx,Priya91/corefx-1,billwert/corefx,elijah6/corefx,alphonsekurian/corefx,JosephTremoulet/corefx,mmitche/corefx,mazong1123/corefx,ellismg/corefx,zhenlan/corefx,cydhaselton/corefx,gkhanna79/corefx,YoupHulsebos/corefx,ptoonen/corefx,alexperovich/corefx,axelheer/corefx,dhoehna/corefx,ViktorHofer/corefx,adamralph/corefx,BrennanConroy/corefx,jlin177/corefx,dotnet-bot/corefx,ViktorHofer/corefx,dsplaisted/corefx,billwert/corefx,rjxby/corefx,elijah6/corefx,khdang/corefx,mmitche/corefx,tijoytom/corefx,tijoytom/corefx,seanshpark/corefx,the-dwyer/corefx,rubo/corefx,Ermiar/corefx,nbarbettini/corefx,SGuyGe/corefx,axelheer/corefx,manu-silicon/corefx,mmitche/corefx,jlin177/corefx,krk/corefx,stephenmichaelf/corefx,nchikanov/corefx,iamjasonp/corefx,ravimeda/corefx,Ermiar/corefx,ptoonen/corefx,ericstj/corefx,krytarowski/corefx,rjxby/corefx,billwert/corefx,tijoytom/corefx,cydhaselton/corefx,ericstj/corefx,jlin177/corefx,wtgodbe/corefx,YoupHulsebos/corefx,weltkante/corefx,ravimeda/corefx,lggomez/corefx,rahku/corefx,mazong1123/corefx,Jiayili1/corefx,ericstj/corefx,yizhang82/corefx,Jiayili1/corefx,marksmeltzer/corefx,Petermarcu/corefx,fgreinacher/corefx,seanshpark/corefx,krk/corefx,Ermiar/corefx,billwert/corefx,Petermarcu/corefx,ViktorHofer/corefx,Chrisboh/corefx,krytarowski/corefx,iamjasonp/corefx,rahku/corefx,YoupHulsebos/corefx,gkhanna79/corefx,seanshpark/corefx,MaggieTsang/corefx,dhoehna/corefx,mmitche/corefx,nchikanov/corefx,khdang/corefx,shimingsg/corefx,lggomez/corefx,khdang/corefx,tstringer/corefx,SGuyGe/corefx,nbarbettini/corefx,marksmeltzer/corefx,krk/corefx,ptoonen/corefx,zhenlan/corefx,mmitche/corefx,Priya91/corefx-1,cydhaselton/corefx,richlander/corefx,Jiayili1/corefx,axelheer/corefx,JosephTremoulet/corefx,twsouthwick/corefx,rahku/corefx,nchikanov/corefx,shimingsg/corefx,Ermiar/corefx,stone-li/corefx,alphonsekurian/corefx,mmitche/corefx