Merge branch 'ps/tests-with-ref-files-backend'

Prepare existing tests on refs to work better with non-default
backends.

* ps/tests-with-ref-files-backend:
  t: mark tests regarding git-pack-refs(1) to be backend specific
  t5526: break test submodule differently
  t1419: mark test suite as files-backend specific
  t1302: make tests more robust with new extensions
  t1301: mark test for `core.sharedRepository` as reffiles specific
  t1300: make tests more robust with non-default ref backends