tests: Make it easier to single-step the msgfmt-* tests.
* gettext-tools/tests/msgfmt-desktop-1: Assign a default to MSGFMT.
* gettext-tools/tests/msgfmt-desktop-2: Likewise.
* gettext-tools/tests/msgfmt-desktop-3: Likewise.
* gettext-tools/tests/msgfmt-xml-1: Likewise.
* gettext-tools/tests/msgfmt-xml-2: Likewise.
* gettext-tools/tests/msgfmt-xml-3: Likewise.
* gettext-tools/tests/msgfmt-xml-4: Likewise.
* gettext-tools/tests/msgfmt-xml-5: Likewise.
