Fix validation of allowed personal backend
Added unit tests for when adding ext storage mount points

- test config was written correctly (global and local)
- test that personal mount points can be added for allowed backends
- test that personal mount points cannot be added for disallowed backends
- added $skipTest flag to make it possible add mount points without
  doing an actual check/connection (note: this is necessary because the
  static class OC_Mount_Config cannot be mocked)
Merge pull request #7792 from owncloud/extstorage-fixpersonalstorage

Fix validation of allowed personal backend