[8.x] Adds "setUpTestDatabase" support to Parallel Testing

[nunomaduro]

This pull request adds setUpTestDatabase support to Parallel Testing. Since we have added Parallel Testing to Laravel, some people have reported that there is no easy way to seed test databases. With this pull request, people can seed their test databases like so:

<?php

namespace App\Providers;

use Illuminate\Support\Facades\ParallelTesting;
use Illuminate\Support\ServiceProvider;

class AppServiceProvider extends ServiceProvider
{
    /**
     * Bootstrap any application services.
     *
     * @return void
     */
    public function boot()
    {
        ParallelTesting::setUpTestDatabase(function () {
            Artisan::call('db:seed');
        });
    }
}

Of course, this hook only gets executed when the test database is created. So, if people want to re-seed their test databases they will need to call art test --parallel --recreate-databases.

@introwit @daniloesser @johanvanhelden Please let me know what do you think.

[nunomaduro]

Please keep in mind that using something like Artisan::call('db:seed'); only makes sense if you are using DatabaseTransactions in your entire test suite.

[BertvanHoekelen]

Thank you @nunomaduro!

[nunomaduro]

@BertvanHoekelen Can you pull this branch locally and give this a try?

[BertvanHoekelen]

@nunomaduro Just tried. Works like a charm! First time database gets seeded second time it skips the seeding. Awesome!
When this gets merged I can finally ditch my custom seed migration :)

[johanvanhelden]

@nunomaduro I pulled the branch and can also confirm it is working flawlessly for my custom bootstrapper! Thank you so much Nuno!

[daniloesser]

@nunomaduro , I can also confirm that the feature is working well! Thank you very much for this improvement!

[taylorotwell]

@nunomaduro could you submit a documentation PR for this? Thanks!