Gemfile: Add the test group not to install rdoc in CI. #906

Job	Run time
openssl-1.0.2u	2m 19s
openssl-1.1.0l	1m 27s
openssl-1.1.1w	1m 38s
openssl-3.0.12	2m 49s
openssl-3.1.4	2m 48s
openssl-3.2.0	3m 39s
libressl-3.1.5	2m 1s
libressl-3.2.7	2m 28s
libressl-3.3.6	2m 8s
libressl-3.4.3	2m 14s
libressl-3.5.3	2m 14s
libressl-3.6.3	2m 12s
libressl-3.7.3	2m 11s
libressl-3.8.1	2m 7s
openssl-3.0.12 fips	2m 42s
openssl-3.1.4 fips	2m 42s
openssl-3.2.0 fips	3m 22s
openssl-head	3m 29s
openssl-head fips	3m 22s
ruby-versions / Generate Ruby versions	3s
ubuntu-22.04 2.7	52s
ubuntu-22.04 3.0	49s
ubuntu-22.04 3.1	56s
ubuntu-22.04 3.2	53s
ubuntu-22.04 head	53s
ubuntu-22.04 truffleruby	1m 23s
ubuntu-22.04 truffleruby-head	1m 33s
ubuntu-20.04 2.7	51s
ubuntu-20.04 3.0	51s
ubuntu-20.04 3.1	47s
ubuntu-20.04 3.2	47s
ubuntu-20.04 head	48s
ubuntu-20.04 truffleruby-head	1m 29s
macos-latest 2.7	1m 40s
macos-latest 3.0	54s
macos-latest 3.1	1m 18s
macos-latest 3.2	55s
macos-latest head	51s
macos-latest truffleruby-head	1m 31s
windows-latest 2.7	4m 19s
windows-latest 3.0	3m 52s
windows-latest 3.1	3m 55s
windows-latest 3.2	4m 59s
windows-latest ucrt	3m 27s
windows-latest mswin	2m 56s
	1h 31m 24s

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