Skip an unstable Ractor test

Author: k0kubun

bootstraptest/test_ractor.rb

@@ -1601,7 +1601,7 @@ class C
 
   1_000.times { idle_worker, tmp_reporter = Ractor.select(*workers) }
   "ok"
-} unless yjit_enabled? || rjit_enabled? # flaky
+} if false # too flaky
 
 assert_equal "ok", %q{
   def foo(*); ->{ super }; end