[ruby/etc] Attempt to re-enable TestEtc#test_ractor_parallel

* If this fails, please revert this commit in ruby/etc
  to keep the repos sync'ed.

https://github.com/ruby/etc/commit/87fb0c4a83

test/etc/test_etc.rb

@@ -175,7 +175,6 @@ class TestEtc < Test::Unit::TestCase
 
   # All Ractor-safe methods should be tested here
   def test_ractor_parallel
-    omit "This test fails randomly with no message; Ractor is not stable yet"
     assert_ractor(<<~RUBY, require: 'etc', timeout: 60)
       10.times.map do
         Ractor.new do