tests: Add more unit tests regarding Desktop files.
Reported by Felix Riemann at <https://savannah.gnu.org/bugs/?51177>
and by Daniel <daniel.rf710@gmail.com>
at <https://savannah.gnu.org/bugs/?63455>.
* gettext-tools/tests/xgettext-desktop-3: New file.
* gettext-tools/tests/msgfmt-desktop-4: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add them.
