This allows to figure out if a test will generate an image without
having to run it. The long term plan for this is to run the tests during
compiling ("make") and not during configuring ("cmake"). Since the list
of files to e.g. install needs to be known during configuring, this
commit is a necessary step in that direction.
Signed-off-by: Uli Schlachter <psychon@znc.in>