Use released version of test-unit-ruby-core #329

Job	Run time
ruby-versions / Generate Ruby versions	2s
test (2.5, ubuntu-latest)	37s
test (2.5, macos-latest)	1m 32s
test (2.5, windows-latest)	2m 17s
test (2.6, ubuntu-latest)	39s
test (2.6, macos-latest)	1m 36s
test (2.6, windows-latest)	4m 33s
test (2.7, ubuntu-latest)	52s
test (2.7, macos-latest)	1m 19s
test (2.7, windows-latest)	2m 52s
test (3.0, ubuntu-latest)	28s
test (3.0, macos-latest)	2m 45s
test (3.0, windows-latest)	2m 38s
test (3.1, ubuntu-latest)	27s
test (3.1, macos-latest)	52s
test (3.1, windows-latest)	4m 9s
test (3.2, ubuntu-latest)	23s
test (3.2, macos-latest)	57s
test (3.2, windows-latest)	2m 6s
test (head, ubuntu-latest)	25s
test (head, macos-latest)	48s
test (head, windows-latest)	3m 24s
test (jruby, ubuntu-latest)	6m 21s
test (jruby-head, ubuntu-latest)	6m 46s
test (truffleruby, ubuntu-latest)	3m 38s
test (truffleruby-head, ubuntu-latest)	3m 48s
	56m 14s

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