[8.x] Allow using dot syntax for $responseKey

[paulandroshchuk]

I just noticed that the assertJsonMissingValidationErrors() assertion does not appreciate the dot syntax of the $responseKey parameter. However, the assertJsonValidationErrors() assertion appreciates that.

So if we had a Laravel project with a customised way of exposing JSON validation errors as something like this:

// customised
'message' => $exception->getMessage(),
'data' => [
  'errors' => $exception->errors(),
],

// default
'message' => $exception->getMessage(),
'errors' => $exception->errors(),

the following test would pass:

$this->postJson('/api/users')
    ->assertJsonValidationErrors(
        ['email' => 'The email field is required.'], 
        $responseKey = 'data.errors',
    )

    // This one will pass as the assertion does not appreciate the dot syntax of the $responseKey parameter
    // as it will try to get this: `$jsonResponse['validation.errors']` rather than `$jsonResponse['validation']['errors']`.
    ->assertJsonMissingValidationErrors(
        ['email'],
        $responseKey = 'data.errors',
    );

So, this PR makes it so it appreciates the dot syntax for the $responseKey parameter on the assertJsonMissingValidationErrors() assertion as it's partially related assertion assertJsonValidationErrors() appreciates that.

[taylorotwell]

This is going to make the error message much more vague because you changed this to a plain assertFalse with no additional message information.

[paulandroshchuk]

Updated.