Switch to use test-unit #394

Job	Run time
ruby-versions / Generate Ruby versions	4s
test (ubuntu-latest, 2.3)	27s
test (ubuntu-latest, 2.4)	28s
test (ubuntu-latest, 2.5)	29s
test (ubuntu-latest, 2.6)	29s
test (ubuntu-latest, 2.7)	31s
test (ubuntu-latest, 3.0)	17s
test (ubuntu-latest, 3.1)	18s
test (ubuntu-latest, 3.2)	16s
test (ubuntu-latest, 3.3)	15s
test (ubuntu-latest, head)	17s
test (ubuntu-latest, jruby)	36s
test (ubuntu-latest, jruby-head)	35s
test (macos-latest, 2.3)	24s
test (macos-latest, 2.4)	24s
test (macos-latest, 2.5)	26s
test (macos-latest, 2.6)	21s
test (macos-latest, 2.7)	24s
test (macos-latest, 3.0)	25s
test (macos-latest, 3.1)	21s
test (macos-latest, 3.2)	28s
test (macos-latest, 3.3)	26s
test (macos-latest, head)	23s
test (macos-latest, jruby)	35s
test (macos-latest, jruby-head)	36s
test (windows-latest, 2.3)	21s
test (windows-latest, 2.4)	21s
test (windows-latest, 2.5)	21s
test (windows-latest, 2.6)	20s
test (windows-latest, 2.7)	19s
test (windows-latest, 3.0)	22s
test (windows-latest, 3.1)	22s
test (windows-latest, 3.2)	22s
test (windows-latest, 3.3)	21s
test (windows-latest, head)	21s
	13m 45s

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