Use test-unit-ruby-core gem #239

Job	Run time
ruby-versions / Generate Ruby versions	3s
build (2.5 / ubuntu-latest)	24s
build (2.5 / macos-latest)	44s
build (2.5 / windows-latest)	3m 46s
build (2.6 / ubuntu-latest)	25s
build (2.6 / macos-latest)	1m 17s
build (2.6 / windows-latest)	3m 7s
build (2.7 / ubuntu-latest)	24s
build (2.7 / macos-latest)	1m 12s
build (2.7 / windows-latest)	1m 49s
build (3.0 / ubuntu-latest)	7s
build (3.0 / macos-latest)	26s
build (3.0 / windows-latest)	1m 17s
build (3.1 / ubuntu-latest)	11s
build (3.1 / macos-latest)	18s
build (3.1 / windows-latest)	1m 38s
build (3.2 / ubuntu-latest)	11s
build (3.2 / macos-latest)	17s
build (3.2 / windows-latest)	2m 1s
build (head / ubuntu-latest)	9s
build (head / macos-latest)	18s
build (head / windows-latest)	1m 18s
build (jruby / ubuntu-latest)	41s
build (jruby / macos-latest)	59s
build (jruby-head / ubuntu-latest)	37s
build (jruby-head / macos-latest)	1m 0s
build (truffleruby / ubuntu-latest)	37s
build (truffleruby / macos-latest)	52s
build (truffleruby-head / ubuntu-latest)	30s
build (truffleruby-head / macos-latest)	43s
	27m 21s

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