Disable GCMetric.GCDuration and GCDurationDelta gtests

The GC invoked before asserting that the two metrics are non-zero are
too small. We need to make them consume more time. Otherwise there is
always a possiblity that the GCs take less than a millisecond and hence
the tests fail.

Bug: 391531096
Bug: 343220989
Test: atest art_standalone_runtime_tests:art_standalone_runtime_tests.HeapTest#GCMetrics -- --abi x86_64
Change-Id: I0c3220b57a16fc85429c493c62c5932ac97ea854