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

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

Change-Id: Id636fefb21404b831b6ba32367642a0dbfa174b2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>