Fix thread leak in test #2766

Job	Run time
ruby-versions / Generate Ruby versions	8s
rake test 2.7 (latest reline)	1m 13s
rake test 2.7	50s
vterm-yamatanooroti 2.7 (latest reline)	1m 38s
rake test 3.0 (latest reline)	49s
vterm-yamatanooroti 2.7	1m 10s
vterm-yamatanooroti 3.0 (latest reline)	1m 46s
vterm-yamatanooroti 3.0	55s
rake test 3.0	39s
vterm-yamatanooroti 3.1 (latest reline)	58s
rake test 3.1 (latest reline)	44s
vterm-yamatanooroti 3.1	49s
rake test 3.1	42s
vterm-yamatanooroti 3.2 (latest reline)	47s
rake test 3.2 (latest reline)	1m 4s
vterm-yamatanooroti 3.2	1m 16s
rake test 3.2	39s
vterm-yamatanooroti head (latest reline)	52s
rake test head (latest reline)	55s
vterm-yamatanooroti head	54s
rake test head	57s
rake test truffleruby-head (latest reline)	4m 59s
rake test truffleruby-head	5m 52s
	30m 36s

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