test-file path is not consistent to other directives

[twodrops]

Other directives like image or include use relative path starting from rSt file. Looks like this is the standard.
.. test-file:: uses relative path starting from folder where conf.py is. This behavior is not consistent with the other directives.

Hint: Change to this behavior will break existing builds. Possibly it shall be changed asap or never.
See similar issue: useblocks/sphinx-needs#351