Use test-unit-ruby-core gem #80

Job	Run time
ruby-versions / Generate Ruby versions	3s
build (2.3 / ubuntu-latest)	25s
build (2.3 / macos-latest)	51s
build (2.4 / ubuntu-latest)	34s
build (2.4 / macos-latest)	1m 2s
build (2.5 / ubuntu-latest)	1m 43s
build (2.5 / macos-latest)	51s
build (2.6 / ubuntu-latest)	29s
build (2.6 / macos-latest)	1m 3s
build (2.7 / ubuntu-latest)	40s
build (2.7 / macos-latest)	54s
build (3.0 / ubuntu-latest)	14s
build (3.0 / macos-latest)	1m 52s
build (3.1 / ubuntu-latest)	11s
build (3.1 / macos-latest)	18s
build (3.2 / ubuntu-latest)	8s
build (3.2 / macos-latest)	20s
build (head / ubuntu-latest)	12s
build (head / macos-latest)	13s
build (jruby / ubuntu-latest)	54s
build (jruby / macos-latest)	39s
build (jruby-head / ubuntu-latest)	46s
build (jruby-head / macos-latest)	42s
build (truffleruby / ubuntu-latest)	34s
build (truffleruby / macos-latest)	52s
build (truffleruby-head / ubuntu-latest)	33s
build (truffleruby-head / macos-latest)	1m 22s
	18m 25s

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