test_io.rb: skip busy wait test again

which I tried to avoid skipping this in r65311, but the test seems not
working under some high load of ci.rvm.jp.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65383 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_io.rb

@@ -548,6 +548,7 @@ class TestIO < Test::Unit::TestCase
 
   if have_nonblock?
     def test_copy_stream_no_busy_wait
+      skip "MJIT has busy wait on GC. This sometimes fails with --jit." if RubyVM::MJIT.enabled?
       skip "multiple threads already active" if Thread.list.size > 1
 
       msg = 'r58534 [ruby-core:80969] [Backport #13533]'