Skip show_doc tests if RDoc is not available #3928

Job	Run time
lint	10s
Debug compatibility test	5m 43s
ruby-versions / Generate Ruby versions	4s
vterm-yamatanooroti 2.7 (latest reline)	1m 7s
rake test 2.7 (latest reline)	1m 37s
vterm-yamatanooroti 2.7	1m 43s
rake test 2.7	1m 28s
vterm-yamatanooroti 3.0 (latest reline)	1m 2s
vterm-yamatanooroti 3.0	1m 2s
rake test 3.0 (latest reline)	1m 26s
rake test 3.0	1m 28s
vterm-yamatanooroti 3.1 (latest reline)	1m 17s
vterm-yamatanooroti 3.1	58s
rake test 3.1 (latest reline)	1m 22s
vterm-yamatanooroti 3.2 (latest reline)	1m 1s
rake test 3.1	1m 27s
vterm-yamatanooroti 3.2	57s
rake test 3.2 (latest reline)	1m 27s
rake test 3.2	1m 23s
vterm-yamatanooroti 3.3 (latest reline)	1m 7s
rake test 3.3 (latest reline)	1m 29s
vterm-yamatanooroti 3.3	56s
rake test 3.3	1m 30s
vterm-yamatanooroti head (latest reline)	1m 6s
rake test head (latest reline)	1m 38s
vterm-yamatanooroti head	57s
rake test head	1m 30s
rake test truffleruby-head (latest reline)	2m 49s
rake test truffleruby-head	1m 3s
	40m 47s

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