[11.x] Allow parallel testing for multiple connections

[lonnylot]

Add support for parallel testing with multiple connections.

Framework users can override the options resolver to return the connections they intend to test on:

ParallelTesting::resolveOptionsUsing(function ($option) {
    if ($option === 'connections') {
        return [ 'mysql', 'pgsql' ];
    }

    $option = 'LARAVEL_PARALLEL_TESTING_'.Str::upper($option);

    return $_SERVER[$option] ?? false;
});

[crynobone]

@lonnylot can you provide a demo application to test this out?

[lonnylot]

@crynobone https://github.com/lonnylot/paralell-testing-demo

You have to run: ./vendor/bin/sail artisan migrate --env=testing

You can try with and without this patch.

EDIT: I have to admit that I developed this patch using 10.x which has CreatesApplication as a trait. This trait was removed in 11.x laravel/laravel#6310 and I'm just realizing now. I still think it makes the most sense and we may want to consider adding it back as a trait because this is a good example of why some people may modify it.

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If possible, please consider releasing your code as a package so that the community can still take advantage of your contributions!

If you feel absolutely certain that this code corrects a bug in the framework, please "@" mention me in a follow-up comment with further explanation so that GitHub will send me a notification of your response.