tests: Reorganize msgmerge tests.
* gettext-tools/tests/msgmerge-9: Renamed from gettext-tools/tests/msgmerge-18.
* gettext-tools/tests/msgmerge-10: Renamed from gettext-tools/tests/msgmerge-19.
* gettext-tools/tests/msgmerge-11: Renamed from gettext-tools/tests/msgmerge-20.
* gettext-tools/tests/msgmerge-12: Renamed from gettext-tools/tests/msgmerge-25.
* gettext-tools/tests/msgmerge-13: Renamed from gettext-tools/tests/msgmerge-26.
* gettext-tools/tests/msgmerge-14: Renamed from gettext-tools/tests/msgmerge-27.
* gettext-tools/tests/msgmerge-15: Renamed from gettext-tools/tests/msgmerge-29.
* gettext-tools/tests/Makefile.am (TESTS): Update.
