Manually register system class aliases in test bootstrap

[jaxwilko]

We register backwards compatible aliases for test cases, however the aliases are not registered in the test bootstrapping.

// modules/system/aliases.php
'TestCase'       => System\Tests\Bootstrap\TestCase::class,
'PluginTestCase' => System\Tests\Bootstrap\PluginTestCase::class,

This PR registers the aliases ahead of time allowing tests to use:

class MyTestCase extends \TestCase

Which will allow for the same test cases to work in both v1.1.9 & >v1.2.0

[what-the-diff]

• The aliases.php file is included in the bootstrap/app.php
• All classes are registered as an alias for autoloading