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