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'

test/test_go.rb

@@ -1853,6 +1853,7 @@ module TestShell
   def test_ctrl_r_multiline
     tmux.send_keys 'echo "foo', :Enter, 'bar"', :Enter
     tmux.until { |lines| lines[-2..-1] == ['foo', 'bar'] }
+    tmux.prepare
     tmux.send_keys 'C-r'
     tmux.until { |lines| lines[-1] == '>' }
     tmux.send_keys 'foo bar'