[RFE] enable shorter test names in plans

[kkaarreell]

I had a test /setup/configure-repo-with-rpms-from-brew' defined in /setup/configure-repo-with-rpms-from-brew/main.fmf. I am using this test in my tmt plans.
Later, I wanted to introduced a parametrized version of this task that would be configuring swtpm repo. I have created /setup/configure-repo-with-rpms-from-brew/swtpm.fmf but from that moment /setup/configure-repo-with-rpms-from-brew stopped being a leaf object and therefore test /setup/configure-repo-with-rpms-from-brew does not exist. To restore original test I have defined /default/ in /setup/configure-repo-with-rpms-from-brew/main.fmf`, so I have tests:

/setup/configure-repo-with-rpms-from-brew/default
/setup/configure-repo-with-rpms-from-brew/swtpm

which I can use in my plans, however I need to update former place to use /setup/configure-repo-with-rpms-from-brew/default instead of /setup/configure-repo-with-rpms-from-brew.

It would be nice to be able to avoid such compatibility breakage and enable using /setup/configure-repo-with-rpms-from-brew in a tmt plan that would be automatically understood as /setup/configure-repo-with-rpms-from-brew/default if /setup/configure-repo-with-rpms-from-brew is not a leaf object.

Just idea, not pushing. :-) I did my plan updates already.