Add parent directory to search files for tests

This add the parent directory as TEXINPUTS search directories for the tests. With that one can launch a `make` in tests without having the Jülich Beamertheme installed globally (which I don't). I don't know why the "$(CURDIR)/src/" is in there…
1 job for fix--test-makefile-curdir in 3 minutes and 37 seconds