run-libcore-tests.sh: reduce explicit GC chatter further

Move explicit GC suppression so it is only applied in gcstress and
also set the GC log threshold higher since this overrides the explicit
GC supression and we still see many messages.

Bug: 161420453
Test: art/tools/run-libcore-tests.sh --mode=host --variant=X32 \
  --debug -- libcore.java.math#test_Constructor_IILjava_util_Random
Change-Id: I1fab3b3571f4ee330ac094ed364bcfcc32fd318f
1 file changed