Skip test_06_timeout for --jit-wait CI

This test has been too unstable on trunk-mjit-wait
http://ci.rvm.jp/results/trunk-mjit-wait@silicon-docker/2782206

test/drb/drbtest.rb

@@ -215,6 +215,7 @@ module DRbCore
   def test_06_timeout
     skip if RUBY_PLATFORM.include?("armv7l-linux")
     skip if RUBY_PLATFORM.include?("sparc-solaris2.10")
+    skip if RubyVM::MJIT.enabled? # expecting a certain delay is difficult for --jit-wait CI
     Timeout.timeout(60) do
       ten = Onecky.new(10)
       assert_raise(Timeout::Error) do