Merge pull request #59 from ruby/use-test-unit-ruby-core #81

Job	Run time
ruby-versions / Generate Ruby versions	4s
build (2.3 / ubuntu-latest)	33s
build (2.3 / macos-latest)	1m 4s
build (2.4 / ubuntu-latest)	26s
build (2.4 / macos-latest)	1m 10s
build (2.5 / ubuntu-latest)	33s
build (2.5 / macos-latest)	57s
build (2.6 / ubuntu-latest)	32s
build (2.6 / macos-latest)	1m 4s
build (2.7 / ubuntu-latest)	24s
build (2.7 / macos-latest)	1m 7s
build (3.0 / ubuntu-latest)	12s
build (3.0 / macos-latest)	15s
build (3.1 / ubuntu-latest)	12s
build (3.1 / macos-latest)	21s
build (3.2 / ubuntu-latest)	11s
build (3.2 / macos-latest)	24s
build (head / ubuntu-latest)	17s
build (head / macos-latest)	16s
build (jruby / ubuntu-latest)	56s
build (jruby / macos-latest)	1m 9s
build (jruby-head / ubuntu-latest)	45s
build (jruby-head / macos-latest)	36s
build (truffleruby / ubuntu-latest)	25s
build (truffleruby / macos-latest)	1m 5s
build (truffleruby-head / ubuntu-latest)	41s
build (truffleruby-head / macos-latest)	42s
	16m 21s

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