Fix the GC total memory test.
Fix the GC total memory test.

This test was relying upon the result of GC.GetTotalMemory() returning a
greater number after allocating a large array. However, the array was
not kept live past the call to GC.GetTotalMemory, which resulted in this
test failing under GCStress=0xC (in which a GC is performed after each
instruction). This change adds the requisite call to GC.KeepAlive to
keep the allocated array live enough to be observed by
GC.GetTotalMemory.

AlexGhiondea/coreclr,rartemev/coreclr,James-Ko/coreclr,ruben-ayrapetyan/coreclr,ragmani/coreclr,AlexGhiondea/coreclr,gkhanna79/coreclr,alexperovich/coreclr,tijoytom/coreclr,krk/coreclr,tijoytom/coreclr,sagood/coreclr,JosephTremoulet/coreclr,JosephTremoulet/coreclr,dpodder/coreclr,parjong/coreclr,parjong/coreclr,botaberg/coreclr,James-Ko/coreclr,jamesqo/coreclr,mmitche/coreclr,mskvortsov/coreclr,JosephTremoulet/coreclr,russellhadley/coreclr,qiudesong/coreclr,sjsinju/coreclr,sjsinju/coreclr,alexperovich/coreclr,ruben-ayrapetyan/coreclr,alexperovich/coreclr,tijoytom/coreclr,krytarowski/coreclr,kyulee1/coreclr,sagood/coreclr,alexperovich/coreclr,yeaicc/coreclr,hseok-oh/coreclr,krytarowski/coreclr,neurospeech/coreclr,sjsinju/coreclr,kyulee1/coreclr,rartemev/coreclr,poizan42/coreclr,dpodder/coreclr,cshung/coreclr,russellhadley/coreclr,JonHanna/coreclr,AlexGhiondea/coreclr,krk/coreclr,JosephTremoulet/coreclr,sjsinju/coreclr,jamesqo/coreclr,James-Ko/coreclr,wateret/coreclr,jamesqo/coreclr,JosephTremoulet/coreclr,kyulee1/coreclr,tijoytom/coreclr,James-Ko/coreclr,botaberg/coreclr,jamesqo/coreclr,neurospeech/coreclr,sjsinju/coreclr,AlexGhiondea/coreclr,yeaicc/coreclr,alexperovich/coreclr,sagood/coreclr,yizhang82/coreclr,pgavlin/coreclr,mskvortsov/coreclr,yizhang82/coreclr,jamesqo/coreclr,poizan42/coreclr,wtgodbe/coreclr,wateret/coreclr,sjsinju/coreclr,qiudesong/coreclr,rartemev/coreclr,kyulee1/coreclr,cydhaselton/coreclr,JonHanna/coreclr,YongseopKim/coreclr,hseok-oh/coreclr,cmckinsey/coreclr,krytarowski/coreclr,botaberg/coreclr,sagood/coreclr,parjong/coreclr,wtgodbe/coreclr,yeaicc/coreclr,ruben-ayrapetyan/coreclr,cshung/coreclr,jamesqo/coreclr,cydhaselton/coreclr,botaberg/coreclr,wateret/coreclr,yizhang82/coreclr,mskvortsov/coreclr,qiudesong/coreclr,poizan42/coreclr,wtgodbe/coreclr,wtgodbe/coreclr,pgavlin/coreclr,yeaicc/coreclr,ragmani/coreclr,wtgodbe/coreclr,dpodder/coreclr,gkhanna79/coreclr,yizhang82/coreclr,cmckinsey/coreclr,parjong/coreclr,JosephTremoulet/coreclr,krytarowski/coreclr,YongseopKim/coreclr,mmitche/coreclr,sagood/coreclr,JonHanna/coreclr,mskvortsov/coreclr,neurospeech/coreclr,mskvortsov/coreclr,rartemev/coreclr,krytarowski/coreclr,cydhaselton/coreclr,gkhanna79/coreclr,yizhang82/coreclr,pgavlin/coreclr,poizan42/coreclr,ruben-ayrapetyan/coreclr,hseok-oh/coreclr,cshung/coreclr,cmckinsey/coreclr,sagood/coreclr,parjong/coreclr,tijoytom/coreclr,wateret/coreclr,cshung/coreclr,qiudesong/coreclr,kyulee1/coreclr,cydhaselton/coreclr,JonHanna/coreclr,krk/coreclr,cmckinsey/coreclr,cmckinsey/coreclr,mmitche/coreclr,cydhaselton/coreclr,cshung/coreclr,russellhadley/coreclr,hseok-oh/coreclr,russellhadley/coreclr,ruben-ayrapetyan/coreclr,YongseopKim/coreclr,rartemev/coreclr,AlexGhiondea/coreclr,wtgodbe/coreclr,pgavlin/coreclr,YongseopKim/coreclr,pgavlin/coreclr,cshung/coreclr,mmitche/coreclr,James-Ko/coreclr,ragmani/coreclr,krytarowski/coreclr,poizan42/coreclr,yizhang82/coreclr,wateret/coreclr,ragmani/coreclr,botaberg/coreclr,russellhadley/coreclr,JonHanna/coreclr,cydhaselton/coreclr,ruben-ayrapetyan/coreclr,mmitche/coreclr,yeaicc/coreclr,kyulee1/coreclr,krk/coreclr,qiudesong/coreclr,JonHanna/coreclr,tijoytom/coreclr,yeaicc/coreclr,mmitche/coreclr,James-Ko/coreclr,rartemev/coreclr,cmckinsey/coreclr,gkhanna79/coreclr,poizan42/coreclr,YongseopKim/coreclr,pgavlin/coreclr,gkhanna79/coreclr,ragmani/coreclr,neurospeech/coreclr,russellhadley/coreclr,hseok-oh/coreclr,gkhanna79/coreclr,krk/coreclr,YongseopKim/coreclr,yeaicc/coreclr,AlexGhiondea/coreclr,dpodder/coreclr,botaberg/coreclr,mskvortsov/coreclr,cmckinsey/coreclr,dpodder/coreclr,dpodder/coreclr,neurospeech/coreclr,parjong/coreclr,ragmani/coreclr,qiudesong/coreclr,hseok-oh/coreclr,wateret/coreclr,neurospeech/coreclr,krk/coreclr,alexperovich/coreclr