Exclude stubbed classes from subclasses after teardown #827

Job	Run time
Rubocop	28s
Ruby 2.7 (Windows)	3m 31s
Legacy Ruby Builds (2.1.9)	1m 11s
Ruby 3.3	2m 31s
Legacy Ruby Builds (2.0)	1m 20s
Ruby 2.6 (Windows)	3m 44s
Legacy Ruby Builds (1.9.3)	1m 8s
Ruby 3.2	2m 15s
Ruby 2.5 (Windows)	2m 7s
Legacy Ruby Builds (1.9.2)	1m 41s
Ruby 3.1	2m 22s
Ruby 2.4 (Windows)	2m 45s
Legacy Ruby Builds (1.8.7)	1m 8s
Legacy Ruby Builds (REE)	1m 18s
Ruby 3.0	2m 43s
Ruby 2.3 (Windows)	2m 30s
Ruby 2.7	3m 43s
Legacy Ruby Builds (JRuby 1.7)	3m 41s
Ruby 2.2 (Windows)	2m 1s
Legacy Ruby Builds (JRuby 1.7 1.8 mode)	3m 7s
Ruby 2.6	3m 49s
Legacy Ruby Builds (JRuby 9.1.17.0)	4m 12s
Ruby 2.5	3m 44s
Ruby 2.4	2m 24s
Ruby 2.3	2m 17s
Ruby 2.2	2m 39s
Ruby ruby-head	32s
Ruby jruby-9.2.13.0	6m 21s
Ruby 2.7 with diff-lcs 1.3	3m 42s
Ruby 2.7 with diff-lcs 1.4.3	3m 44s
	1h 18m 38s

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