[rubygems/rubygems] Improve Gem::SpecFetcher tests. #671

Job	Run time
make (check, cppflags=-DVM_CHECK_MODE)	8m 8s
make (check, i686)	11m 31s
make (check, --disable-yjit)	6m 54s
make (check, --enable-shared --enable-load-relative)	7m 19s
make (check, true, /home/runner/ruby_gc, --with-shared-gc=/home/runner/ruby_gc)	9m 11s
make (test-bundler-parallel, 50)	27m 15s
make (test-bundled-gems)	16m 30s
make (check, ubuntu-20.04)	9m 52s
make (check, ubuntu-24.04)	9m 11s
Ubuntu result	1s
	1h 45m 52s

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