Restrict version of minitest to avoid test errors #191

Job	Run time
Unit test (ubuntu-20.04, 2.1.9)	1m 8s
Unit test (ubuntu-20.04, 2.2.10)	1m 18s
Unit test (ubuntu-20.04, 2.3.8)	1m 12s
Unit test (ubuntu-20.04, 2.4.6)	1m 9s
Unit test (ubuntu-20.04, 2.5.8)	40s
Unit test (ubuntu-20.04, 2.6.6)	39s
Unit test (ubuntu-20.04, 2.7.2)	37s
Unit test (ubuntu-20.04, 3.0.1)	21s
Unit test (ubuntu-20.04, 3.1)	22s
Unit test (ubuntu-20.04, 3.2)	20s
Unit test (ubuntu-20.04, jruby-9.1.17.0)	1m 14s
Unit test (ubuntu-20.04, jruby-9.2.17.0)	1m 32s
Unit test (ubuntu-20.04, jruby-9.3.2.0)	58s
Unit test (ubuntu-20.04, jruby-9.4.0.0)	55s
Unit test (ubuntu-20.04, truffleruby)	2m 28s
Unit test (macos-latest, 2.1.9)	2m 26s
Unit test (macos-latest, 2.2.10)	2m 50s
Unit test (macos-latest, 2.3.8)	2m 43s
Unit test (macos-latest, 2.4.6)	4m 36s
Unit test (macos-latest, 2.5.8)	1m 9s
Unit test (macos-latest, 2.6.6)	1m 12s
Unit test (macos-latest, 2.7.2)	3m 0s
Unit test (macos-latest, 3.0.1)	50s
Unit test (macos-latest, 3.1)	44s
Unit test (macos-latest, 3.2)	55s
Unit test (macos-latest, jruby-9.1.17.0)	1m 32s
Unit test (macos-latest, jruby-9.2.17.0)	1m 46s
Unit test (macos-latest, jruby-9.3.2.0)	3m 10s
Unit test (macos-latest, jruby-9.4.0.0)	1m 29s
Unit test (macos-latest, truffleruby)	9m 38s
finish	0s
	52m 53s

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