Lock minitest to < 6 #288

Job	Run time
tests (3.3, 8.1)	5m 45s
tests (3.2, 8.0)	5m 44s
tests (3.3, edge)	12s
tests (4.0, edge)	11s
tests (3.4, edge)	9s
tests (4.0, 7.1)	5m 11s
tests (3.2, 8.1)	5m 11s
tests (3.4, 8.0)	5m 30s
tests (head, 7.1)	8s
tests (3.4, 7.2)	5m 48s
tests (4.0, 7.2)	5m 44s
tests (3.3, 7.2)	5m 45s
tests (4.0, 8.0)	5m 42s
tests (3.1, 7.2)	5m 39s
tests (3.2, 7.2)	5m 48s
tests (3.4, 7.1)	5m 24s
tests (3.3, 8.0)	5m 54s
tests (3.4, 8.1)	5m 46s
tests (head, 7.2)	12s
tests (head, edge)	15s
tests (head, 8.1)	12s
tests (3.1, 7.1)	4m 58s
tests (head, 8.0)	10s
tests (4.0, 8.1)	5m 29s
tests (3.3, 7.1)	5m 39s
tests (3.2, 7.1)	5m 13s
tests (3.3, 7.1, --enable-frozen-string-literal)	5m 28s
	1h 47m 7s

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