tests: allow having the test/reference directory in another subdir
tests: allow having the test/reference directory in another subdir

Useful for external projects, like anaconda-webui, where the cockpit
code is not in the root of the repo.
cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit