build: Fix Makefiles for portability.

* tests/gpg/Makefile.am: Don't allow target with '/'.
* tests/gpgsm/Makefile.am: Ditto.

BSD Make doesn't allow a target with '/'. We still have such a target
in lang/python/Makefile.am, but it's for maintainer only, so,
assumption to GNU Make is OK there.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>