Use released version of test-unit-ruby-core #328

Job	Run time
ruby-versions / Generate Ruby versions	1s
test (2.5, ubuntu-latest)	12s
test (2.5, macos-latest)	7s
test (2.5, windows-latest)	10s
test (2.6, ubuntu-latest)	13s
test (2.6, macos-latest)	3s
test (2.6, windows-latest)	11s
test (2.7, ubuntu-latest)	11s
test (2.7, macos-latest)	9s
test (2.7, windows-latest)	1s
test (3.0, ubuntu-latest)	26s
test (3.0, macos-latest)	8s
test (3.0, windows-latest)	11s
test (3.1, ubuntu-latest)	28s
test (3.1, macos-latest)	9s
test (3.1, windows-latest)	8s
test (3.2, ubuntu-latest)	10s
test (3.2, macos-latest)	7s
test (3.2, windows-latest)	1m 29s
test (head, ubuntu-latest)	26s
test (head, macos-latest)	8s
test (head, windows-latest)	1s
test (jruby, ubuntu-latest)	13s
test (jruby-head, ubuntu-latest)	13s
test (truffleruby, ubuntu-latest)	14s
test (truffleruby-head, ubuntu-latest)	14s
	6m 3s

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