Allow a package maintainer to define path to the test-cases repository

Downstreams can have different instances for apertis-test-cases with different image repositories and supported boards. Thus, a package maintainer should be able to select which instance is needed to test its package.

Fix: apertis-issues#527 (closed)

With this MR, it is possible to define test_cases_path in debian/apertis/package-centric-testing.yaml like:

  test_cases_path: tests/apertis-test-cases

This variable doesn't contain the full url since we need to inject the ci token in it for private project, thus a package maintainer can only define the final part of the url (i.e. the url of gitlab project).