Skip TestEtc#test_ractor_parallel on ModGC workflow (#55)

https://bugs.ruby-lang.org/issues/21204

TestEtc#test_ractor_parallel is only failing intermittently on ModGC workflow after 87fb0c4. So, we'll skip this test on ModGC workflow.

Author: ono-max

test/etc/test_etc.rb

@@ -175,6 +175,8 @@ def test_sysconfdir
 
   # All Ractor-safe methods should be tested here
   def test_ractor_parallel
+    omit "This test is flaky and intermittently failing now on ModGC workflow" if ENV['GITHUB_WORKFLOW'] == 'ModGC'
+
     assert_ractor(<<~RUBY, require: 'etc', timeout: 60)
       10.times.map do
         Ractor.new do