[6.x] Add withoutMix and withMix test helpers

[jasonmccreary]

Similar to withoutExceptionHandling and withExceptionHandling, these two test helpers allow developers to quickly unbinding and rebinding Laravel Mix in the container during testing.

This behavior was added in #26289, but still requires intimate knowledge of the underlying Mix class and proper binding function. Furthermore, it adds a relatively obscure, big block of code for a simple need.

Before

$this->swap(\Illuminate\Foundation\Mix::class, function () {
    return '';
});

After

$this->withoutMix();

Note: Inline with other TestCase methods, withoutMix returns the instance allowing it to be chained with other fluent methods, e.g. $this->withoutMix()->withoutExceptionHandling()->get('/');

[driesvints]

Would be cool if we could somehow implement something that disables mix all together for all tests. Like calling this method in the setUp method of the TestCase class.

[driesvints]

Turns out this doesn't works when using browser kit testing's visit method. Unfortunate but understandable.

[carbontwelve]

This has proved necessary in a project I just took over with "six mysteriously failing unit tests that pass on our locals." It would have been super helpful if this was documented on https://github.com/laravel/docs/blob/8.x/testing.md