tests: Extract tests from dist-git source

This avoids a hardcoded `ref:` which needs to be updated on every
upstream release, and having to pull the tests from an outside place.

See https://tmt.readthedocs.io/en/stable/spec/plans.html

plans/upstream.fmf

@@ -1,7 +1,5 @@
 discover:
   how: fmf
-  repository: https://github.com/cockpit-project/cockpit
+  dist-git-source: true
-  # FIXME: get rid of the hardcoding: https://github.com/psss/tmt/issues/585
-  ref: "273"
 execute:
   how: tmt