UnitTests - Tighten class-scanning for `civicrm-core:tests/phpunit`

UnitTests - Tighten class-scanning for `civicrm-core:tests/phpunit`

This addresses a bug where:

1. Class-scanning of `civicrm-core:tests/phpunit/*` works with core-test-suites
2. Class-scanning of `civicrm-core:tests/phpunit/*` fails with civiimport test-suite.

The difference is that core-tests setup class-loading for
`civicrm-core:tests/phpunit/`; `civimport` understandably doesn't.

Class-scanning is only viable if we regard the specific classes a loadable.

This further tightens it:

1. Since core-test-suites can load `civicrm-core:tests/phpunit/*`, they will scan it
2. Since civiimport test-suite cannot load it, it will not scan it.