Skip test_wait on MinGW CI

This test has been unstable, and it seems like we're not interested in
fixing that for MinGW.
https://github.com/ruby/ruby/actions/runs/3073317191/jobs/4965373284

test/io/wait/test_io_wait.rb

@@ -50,6 +50,7 @@ class TestIOWait < Test::Unit::TestCase
   end
 
   def test_wait
+    omit 'unstable on MinGW' if /mingw/ =~ RUBY_PLATFORM
     assert_nil @r.wait(0)
     @w.syswrite "."
     sleep 0.1