tests: Make portability fix.

* lang/python/tests/Makefile.am: Distinguish target and path.
* tests/gpg/Makefile.am: Ditto.
* tests/gpgsm/Makefile.am: Ditto.

GNU Make is powerful enough to match path to target (and vice versa),
but BSD make is not.

• GnuPG-bug-id: T3056
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>