run-libcore-tests.sh: add --gcstress command line flag

Simplify selecting gcstress mode, previously required --vm-arg
-Xgc:gcstress.

Test: run-libcore-tests.sh --mode=host --debug --gcstress
Test: run-libcore-tests.sh --mode=host --debug --vm-arg -Xgc:gcstress
Change-Id: I0f99dbc41a374ae578c93f4c0e59173a92c42d16
1 file changed