Improve and simplify error reporting in test `HeapTest.GCMetrics`. am: 92181e6b46 am: 121f4bd0c6 am: cb75c67280

Original change: https://android-review.googlesource.com/c/platform/art/+/2469481

Change-Id: I5ece781d739bb4b486d5e32f730374f435a48e8f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>