Provide common way for app acceptance tests to find their location relative to core

[phil-davis]

Some core acceptance test helper code or test step code needs to know a relative path from the core tests/acceptance folder to some data file that the test needs to upload or compare with or...

If a system-under-test (e.g. in a developer environment or...) has an app-under-test installed in an alternate apps folder - e.g. in apps-external - then the app test code needs to be able to give a relative path to its tests/acceptance folder. The app cannot just hardcode:
../../apps/guests/tests/acceptance

because it might be something like:
../../apps-external/guests/tests/acceptance

• Provide a convenience routine in core that can work out the relative path. That will avoid many apps needing to implement the needed logic. Provide convenience test method getPathFromCoreToAppAcceptanceTests #34366
• call it from guests app Use core getPathFromCoreToAppAcceptanceTests method guests#306
• call it from files_antivirus app Use core getPathFromCoreToAppAcceptanceTests method files_antivirus#292
• call it from files_classifier app https://github.com/owncloud/files_classifier/pull/135

[ownclouders]

GitMate.io thinks possibly related issues are #22485 (test), #22492 (test), #31579 (test), #32804 (Refactor acceptance test methods for accessing testing app), and #32806 (Refactor acceptance test methods for accessing testing app).

[phil-davis]

core changes are merged.
App changes to be done tomorrow morning after the core QA tarballs are available.

[phil-davis]

done