Restore execution of CaseActivityTest

Restore execution of CaseActivityTest

Following eb92dd792c07e0b11ee1561cf00930402345e8b3, the `CaseActivityTest` started to run
only intermittently. Why?

__high-level__: `Civi\Core\ClassScanner` and `phpunit8` both do a scan over the folder `tests/phpunit/CRM/Case/WorkflowMessage`

__low-level__: `Civi\Core\ClassScanner` has caching. Depending on the state of the cache, it may or may not do a scan:

* If the cache is filled, then `ClassScanner` doesn't need to scan.
    * When `phpunit8` subsequently does a scan, it will load `CaseActivityTest.php` normally.
* If the cache is empty, then `ClassScanner` does the first scan. It is the one that actually loads `CaseActivityTest.php`.
    * Later, `phpunit8` does a scan. Due to a quirk, it doesn't realize the class exists.

The scanner in phpunit works roughly like this:

```php
$tests = [];
foreach (glob('*Test.php') as $file) {
  $before = get_declared_classes();
  require_once $file;
  $after = get_declared_classes();
  $tests = array_merge($tests, array_diff($before, $after));
}
```

So if the class was previously loaded, then phpunit doesn't see it.