Assert that a response matches the specified errors

[pacoorozco]

When I'm testing my controller I usually create several test cases to ensure that validation is in place. I usually create a data provider with the form data and the errors that I'm expected. In order to assert if the validation has raised errors I was using assertSessionHasErrors() and assertSessionDoesNotHaveErrors().

But I've created a new assertion that could be interesting to be shared: assertSessionMatchesErrors().

The assertion will pass if the provided errors and only these ones are present in the session.

Let's imagine a form request with 4 fields that are validated: field1, field2, field3 and field4.

If you have a testcase to ensure that field1 validation fails you need to do:

// To ensure that only `field1` is failing.

$this->assertSessionHasErrors(['field1']);
$this->assertSessionDoesNotHaveErrors(['field2', 'field3', 'field4']);

// Instead you can do

$this->assertSessionMatchesErrors(['field1']);

I've created this PR just in case that you can to take a look to my implementation.

Do you think it could be useful?