Skip an unstable test on MinGW

This test fails fairly frequently on MinGW:
https://github.com/ruby/ruby/actions/runs/7195712496/job/19598924253#step:11:168
https://github.com/ruby/ruby/actions/runs/7191246799/job/19585627182#step:11:168

and we aren't actively working on stabilizing tests for MinGW.

test/io/wait/test_io_wait.rb

@@ -36,6 +36,7 @@ class TestIOWait < Test::Unit::TestCase
   end
 
   def test_ready?
+    omit 'unstable on MinGW' if /mingw/ =~ RUBY_PLATFORM
     assert_not_predicate @r, :ready?, "shouldn't ready, but ready"
     @w.syswrite "."
     sleep 0.1