Fix make_shareable shim in harness-common.rb #1496

Job	Run time
test (truffleruby)	19s
test (ruby)	12s
benchmark-default (ruby)	10m 47s
test (head)	11s
benchmark-default (truffleruby)	13m 54s
benchmark-default (head)	11m 8s
benchmark-ractor-only (head)	46s
benchmark-ractor-only (ruby)	55s
benchmark-ractor-only (truffleruby)	11s
benchmark-ractor (ruby)	12m 49s
benchmark-graph	46s
benchmark-ractor (truffleruby)	16s
benchmark-run-once	1m 36s
benchmark-ractor (head)	12m 29s
all-tests	4s
	1h 6m 23s

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