[8.x] Add scheduler integration tests

[inxilpro]

This adds two tests for the scheduler. The first tests scheduled callbacks, and the second tests scheduled commands.

Scheduled callbacks are relatively easy to test because they always run in the same process as the scheduler. These tests verify execution order, exception handling, and mutex processing (for onOneServer).

Scheduled commands are trickier to test because they rely on spawning an external php artisan ... process, which is currently not supported in Orchestra Testbench. To work around this issue, we write a custom artisan script that is used for our test. It includes:

1. A custom console command (which must exist outside of the scope of the test so that it's available to execute in a secondary process)
2. Command and scheduler registration (again, necessary because half the test does not execute inside the context of PHPUnit)
3. A custom version of the typical artisan script that is modified to account for the location of the testbench-core/laravel directory

(This file is removed during the tear down phase.)

The command tests work by writing to a log file and then checking that the expected output exists in that file. This means that we can use the full Laravel scheduler (including the call to php artisan schedule:finish when running in background) without having to mock or fake anything.