[5.5] Add withMiddleware function in MakesHttpRequests

[mathieutu]

Hi.
This PR adds a withMiddleware method, as the inverse of withoutMiddleware one.

Example:

I have a middleware applied to all my application. I want to deactivate this middleware during the tests:

abstract class TestCase extends BaseTestCase
{
    use CreatesApplication, RefreshDatabase;

    protected function setUp()
    {
        parent::setUp();
        
        $this->withoutMiddleware(MyMiddleware::class);
    }
}

But, just in some tests, where the middleware is needed, I would like to re-enable it:

public function testThatMyMiddlewareIsApplied() {
        $this->withMiddleware(MyMiddleware::class);
        // my test
}

It's kind the same than withExceptionHandling method.

It's fully tested, and I've taken this opportunity to add some tests for the withoutMiddleware existing method.

Thanks for your answer.