Test

Add weak_references_count to GC.stat #306

Job	Run time
check (MarkSweep, release, macos-latest)	13m 5s
check (Immix, debug, 150, macos-latest)	50m 5s
check (Immix, debug, 150, ubuntu-latest)	1h 13m 18s
check (Immix, release, macos-latest)	15m 21s
check (MarkSweep, debug, 150, macos-latest)	55m 20s
check (MarkSweep, debug, 150, ubuntu-latest)	1h 6m 49s
check (MarkSweep, release, ubuntu-latest)	10m 35s
check (Immix, release, ubuntu-latest)	12m 7s
	4h 56m 40s

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