Improve and simplify error reporting in test `HeapTest.GCMetrics`.

Test: ART_USE_READ_BARRIER=true ART_USE_GENERATIONAL_CC=true \
        m test-art-host-gtest-art_runtime_tests
Bug: 271112044
Bug: 270391874
Change-Id: I1230b9979eb9bbaade3336f2e2dd1271aab4e4fc
2 files changed