[5.x] Simplify TestCase for addons

[duncanmcclean]

This pull request introduces a new AddonTestCase class which can be used by addon developers to simplify their addon's TestCase.

Currently, addons need to implement a bunch of methods to boot Statamic, boot your addon's service providers & setup the addon's manifest.

Now, with the changes in this PR, the TestCase for most addons can be simplified to:

<?phpnamespaceStatamic\SeoPro\Tests;
useStatamic\SeoPro\ServiceProvider;
useStatamic\Extend\AddonTestCase;
abstractclass TestCase extends AddonTestCase
{
protectedstring$addonServiceProvider = ServiceProvider::class;
}

To Do

• Ensure everything we "magically" load for addons in the AddonServiceProvider also get automatically loaded in tests.