tests: Reorganize msgmerge tests.
* gettext-tools/tests/msgmerge-6: Renamed from gettext-tools/tests/msgmerge-9.
* gettext-tools/tests/msgmerge-7: Renamed from gettext-tools/tests/msgmerge-15.
* gettext-tools/tests/msgmerge-8: Renamed from gettext-tools/tests/msgmerge-17.
* gettext-tools/tests/Makefile.am (TESTS): Update.
