[9.x] Add assertBadRequest to the TestResponse.

[xiCO2k]

This PR adds the assertBadRequest to the TestResponse class.

This will allow to use:

$response->assertBadRequest();

instead of:

$response->assertStatus(400);

This one was also submitted in the past by @michael-rubel on #43047.

[rschoonheim]

Are there any other common response status assertions missing? I assume we want to have a consistent API right?

[taylorotwell]

I don't really want to add a method for every response code. 😬

[xiCO2k]

Got it