Separate harness implementation from the harness framework #1515

Job	Run time
benchmark-default (truffleruby)	15m 6s
benchmark-default (head)	10m 31s
test (ruby)	58s
test (head)	58s
test (truffleruby)	57s
benchmark-default (ruby)	11m 12s
benchmark-graph	54s
benchmark-ractor-only (ruby)	56s
benchmark-ractor (head)	12m 18s
benchmark-run-once	1m 32s
benchmark-ractor (ruby)	13m 2s
benchmark-ractor-only (head)	50s
all-tests	2s
	1h 9m 16s

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