Fix flaky test case

Make sure that the shell is ready before hitting CTRL-R

      1) Error:
    TestFish#test_ctrl_r_multiline:
    RuntimeError: timeout
        test/test_go.rb:50:in `wait'
        test/test_go.rb:125:in `until'
        test/test_go.rb:1857:in `test_ctrl_r_multiline'