Skip BigDecimal tests when it's missing to load #128

Job	Run time
ruby-versions / Generate Ruby versions	1s
ubuntu-20.04 2.3	1m 20s
ubuntu-20.04 2.4	1m 29s
ubuntu-20.04 2.5	1m 38s
ubuntu-20.04 2.6	1m 26s
ubuntu-20.04 2.7	1m 34s
ubuntu-20.04 3.0	47s
ubuntu-20.04 3.1	36s
ubuntu-20.04 3.2	38s
ubuntu-20.04 head	33s
ubuntu-20.04 jruby	1m 29s
ubuntu-20.04 jruby-head	1m 29s
ubuntu-20.04 truffleruby	1m 8s
ubuntu-20.04 truffleruby-head	1m 40s
ubuntu-22.04 2.3	1m 7s
ubuntu-22.04 2.4	1m 10s
ubuntu-22.04 2.5	1m 15s
ubuntu-22.04 2.6	1m 21s
ubuntu-22.04 2.7	1m 20s
ubuntu-22.04 3.0	38s
ubuntu-22.04 3.1	36s
ubuntu-22.04 3.2	34s
ubuntu-22.04 head	34s
ubuntu-22.04 jruby	1m 22s
ubuntu-22.04 jruby-head	1m 28s
ubuntu-22.04 truffleruby	1m 6s
ubuntu-22.04 truffleruby-head	1m 19s
macos-11 2.3	2m 52s
macos-11 2.4	1m 57s
macos-11 2.5	1m 57s
macos-11 2.6	2m 16s
macos-11 2.7	2m 10s
macos-11 3.0	35s
macos-11 3.1	39s
macos-11 3.2	51s
macos-11 head	40s
macos-11 jruby	1m 16s
macos-11 jruby-head	2m 22s
macos-11 truffleruby	2m 49s
macos-11 truffleruby-head	2m 24s
macos-12 2.3	2m 23s
macos-12 2.4	2m 23s
macos-12 2.5	2m 11s
macos-12 2.6	2m 19s
macos-12 2.7	2m 11s
macos-12 3.0	38s
macos-12 3.1	40s
macos-12 3.2	40s
macos-12 head	45s
macos-12 jruby	1m 35s
macos-12 jruby-head	3m 22s
macos-12 truffleruby	2m 17s
macos-12 truffleruby-head	3m 54s
windows-latest 2.3	2m 49s
windows-latest 2.4	3m 29s
windows-latest 2.5	5m 4s
windows-latest 2.6	5m 2s
windows-latest 2.7	4m 6s
windows-latest 3.0	2m 1s
windows-latest 3.1	2m 12s
windows-latest 3.2	2m 53s
windows-latest head	2m 23s
ubuntu-latest jruby-9.1	2m 8s
ubuntu-latest jruby-9.2	2m 20s
ubuntu-latest jruby-9.3	1m 36s
	1h 53m 47s

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