fix intermittent failures in StringTests
fix intermittent failures in StringTests

yizhang82/corefx,yizhang82/corefx,krk/corefx,MaggieTsang/corefx,ptoonen/corefx,cydhaselton/corefx,mazong1123/corefx,twsouthwick/corefx,twsouthwick/corefx,nchikanov/corefx,MaggieTsang/corefx,Ermiar/corefx,nchikanov/corefx,the-dwyer/corefx,stone-li/corefx,seanshpark/corefx,Ermiar/corefx,tijoytom/corefx,richlander/corefx,nbarbettini/corefx,stone-li/corefx,shimingsg/corefx,JosephTremoulet/corefx,krk/corefx,wtgodbe/corefx,JosephTremoulet/corefx,Ermiar/corefx,MaggieTsang/corefx,richlander/corefx,ptoonen/corefx,ViktorHofer/corefx,Ermiar/corefx,MaggieTsang/corefx,shimingsg/corefx,ericstj/corefx,parjong/corefx,DnlHarvey/corefx,zhenlan/corefx,zhenlan/corefx,shimingsg/corefx,rubo/corefx,stone-li/corefx,parjong/corefx,nchikanov/corefx,stone-li/corefx,shimingsg/corefx,mazong1123/corefx,DnlHarvey/corefx,seanshpark/corefx,nbarbettini/corefx,zhenlan/corefx,the-dwyer/corefx,nbarbettini/corefx,Jiayili1/corefx,shimingsg/corefx,zhenlan/corefx,DnlHarvey/corefx,rubo/corefx,richlander/corefx,mmitche/corefx,jlin177/corefx,DnlHarvey/corefx,fgreinacher/corefx,BrennanConroy/corefx,mmitche/corefx,JosephTremoulet/corefx,Ermiar/corefx,krk/corefx,richlander/corefx,wtgodbe/corefx,ravimeda/corefx,parjong/corefx,nchikanov/corefx,jlin177/corefx,shimingsg/corefx,krk/corefx,axelheer/corefx,yizhang82/corefx,ptoonen/corefx,ravimeda/corefx,DnlHarvey/corefx,axelheer/corefx,jlin177/corefx,parjong/corefx,nchikanov/corefx,twsouthwick/corefx,ravimeda/corefx,nbarbettini/corefx,fgreinacher/corefx,yizhang82/corefx,rubo/corefx,DnlHarvey/corefx,jlin177/corefx,ptoonen/corefx,ericstj/corefx,nbarbettini/corefx,zhenlan/corefx,zhenlan/corefx,axelheer/corefx,mmitche/corefx,tijoytom/corefx,nchikanov/corefx,the-dwyer/corefx,axelheer/corefx,ViktorHofer/corefx,twsouthwick/corefx,ViktorHofer/corefx,ptoonen/corefx,cydhaselton/corefx,JosephTremoulet/corefx,tijoytom/corefx,ptoonen/corefx,ericstj/corefx,fgreinacher/corefx,DnlHarvey/corefx,JosephTremoulet/corefx,seanshpark/corefx,Jiayili1/corefx,MaggieTsang/corefx,mazong1123/corefx,axelheer/corefx,ViktorHofer/corefx,Jiayili1/corefx,richlander/corefx,yizhang82/corefx,tijoytom/corefx,cydhaselton/corefx,the-dwyer/corefx,cydhaselton/corefx,zhenlan/corefx,ericstj/corefx,ravimeda/corefx,twsouthwick/corefx,cydhaselton/corefx,mmitche/corefx,ericstj/corefx,BrennanConroy/corefx,twsouthwick/corefx,Jiayili1/corefx,mazong1123/corefx,mazong1123/corefx,shimingsg/corefx,yizhang82/corefx,Ermiar/corefx,mmitche/corefx,ravimeda/corefx,mazong1123/corefx,BrennanConroy/corefx,ericstj/corefx,the-dwyer/corefx,tijoytom/corefx,the-dwyer/corefx,yizhang82/corefx,ViktorHofer/corefx,tijoytom/corefx,seanshpark/corefx,ericstj/corefx,cydhaselton/corefx,MaggieTsang/corefx,cydhaselton/corefx,fgreinacher/corefx,wtgodbe/corefx,stone-li/corefx,ViktorHofer/corefx,rubo/corefx,mmitche/corefx,seanshpark/corefx,nbarbettini/corefx,richlander/corefx,krk/corefx,mmitche/corefx,Ermiar/corefx,parjong/corefx,wtgodbe/corefx,Jiayili1/corefx,wtgodbe/corefx,Jiayili1/corefx,MaggieTsang/corefx,ravimeda/corefx,mazong1123/corefx,krk/corefx,nchikanov/corefx,jlin177/corefx,JosephTremoulet/corefx,seanshpark/corefx,the-dwyer/corefx,parjong/corefx,ViktorHofer/corefx,axelheer/corefx,ravimeda/corefx,ptoonen/corefx,JosephTremoulet/corefx,jlin177/corefx,stone-li/corefx,nbarbettini/corefx,Jiayili1/corefx,wtgodbe/corefx,tijoytom/corefx,seanshpark/corefx,krk/corefx,parjong/corefx,jlin177/corefx,stone-li/corefx,richlander/corefx,twsouthwick/corefx,wtgodbe/corefx,rubo/corefx