Add test for `show_cmds` pager's termination #2209

Job	Run time
ruby-versions / Generate Ruby versions	1s
vterm-yamatanooroti 2.7 (latest reline)	1m 4s
vterm-yamatanooroti 2.7	1m 5s
vterm-yamatanooroti 3.0 (latest reline)	34s
vterm-yamatanooroti 3.0	47s
vterm-yamatanooroti 3.1 (latest reline)	54s
vterm-yamatanooroti 3.1	39s
vterm-yamatanooroti 3.2 (latest reline)	35s
vterm-yamatanooroti 3.2	52s
vterm-yamatanooroti head (latest reline)	40s
vterm-yamatanooroti head	37s
rake test 2.7 (latest reline)	45s
rake test 2.7	55s
rake test 3.0 (latest reline)	30s
rake test 3.0	40s
rake test 3.1 (latest reline)	31s
rake test 3.1	34s
rake test 3.2 (latest reline)	31s
rake test 3.2	27s
rake test head (latest reline)	26s
rake test head	28s
rake test truffleruby-head (latest reline)	5m 15s
rake test truffleruby-head	7m 8s
	25m 58s

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