Add string.Concat(IEnumerable<char>) tests (#24398)
Add string.Concat(IEnumerable<char>) tests (#24398)

Exercise char special paths in string.Concat.
parjong/corefx,axelheer/corefx,parjong/corefx,seanshpark/corefx,fgreinacher/corefx,Ermiar/corefx,shimingsg/corefx,Ermiar/corefx,wtgodbe/corefx,Ermiar/corefx,Jiayili1/corefx,zhenlan/corefx,ericstj/corefx,twsouthwick/corefx,ericstj/corefx,ptoonen/corefx,tijoytom/corefx,ViktorHofer/corefx,fgreinacher/corefx,shimingsg/corefx,axelheer/corefx,ericstj/corefx,axelheer/corefx,axelheer/corefx,wtgodbe/corefx,ViktorHofer/corefx,parjong/corefx,ravimeda/corefx,Jiayili1/corefx,mmitche/corefx,tijoytom/corefx,BrennanConroy/corefx,seanshpark/corefx,axelheer/corefx,tijoytom/corefx,ptoonen/corefx,Jiayili1/corefx,mmitche/corefx,ViktorHofer/corefx,BrennanConroy/corefx,wtgodbe/corefx,Ermiar/corefx,parjong/corefx,twsouthwick/corefx,ravimeda/corefx,seanshpark/corefx,ptoonen/corefx,ViktorHofer/corefx,ericstj/corefx,parjong/corefx,wtgodbe/corefx,ViktorHofer/corefx,shimingsg/corefx,zhenlan/corefx,mmitche/corefx,axelheer/corefx,twsouthwick/corefx,Jiayili1/corefx,fgreinacher/corefx,twsouthwick/corefx,seanshpark/corefx,shimingsg/corefx,ravimeda/corefx,mmitche/corefx,zhenlan/corefx,twsouthwick/corefx,seanshpark/corefx,ViktorHofer/corefx,zhenlan/corefx,Jiayili1/corefx,tijoytom/corefx,shimingsg/corefx,ptoonen/corefx,seanshpark/corefx,twsouthwick/corefx,ericstj/corefx,tijoytom/corefx,ericstj/corefx,BrennanConroy/corefx,zhenlan/corefx,mmitche/corefx,ptoonen/corefx,tijoytom/corefx,ViktorHofer/corefx,ptoonen/corefx,parjong/corefx,Ermiar/corefx,ericstj/corefx,ravimeda/corefx,mmitche/corefx,Ermiar/corefx,ravimeda/corefx,zhenlan/corefx,tijoytom/corefx,Jiayili1/corefx,seanshpark/corefx,twsouthwick/corefx,wtgodbe/corefx,wtgodbe/corefx,Jiayili1/corefx,parjong/corefx,fgreinacher/corefx,zhenlan/corefx,ptoonen/corefx,ravimeda/corefx,wtgodbe/corefx,shimingsg/corefx,mmitche/corefx,ravimeda/corefx,shimingsg/corefx,Ermiar/corefx