Enable mock expectations to set on === method

I have a scenario where I have to validate against a Regex or arbitrary function so I'm passing a `Regex` or lambda and sending `===` to the format object.

I don't see that it's current possible to test this behaviour with a `Minitest::Mock` as you cannot set an expectation for the `===` message.

In this PR I have copied the example set by the `respond_to?` method and made a it a special case.