tests: Reorganize msgmerge tests.
* gettext-tools/tests/msgmerge-domain-1: Renamed from gettext-tools/tests/msgmerge-7.
* gettext-tools/tests/msgmerge-domain-2: Renamed from gettext-tools/tests/msgmerge-8.
* gettext-tools/tests/msgmerge-charset-1: Renamed from gettext-tools/tests/msgmerge-6.
* gettext-tools/tests/msgmerge-charset-2: Renamed from gettext-tools/tests/msgmerge-14.
* gettext-tools/tests/msgmerge-charset-3: Renamed from gettext-tools/tests/msgmerge-16.
* gettext-tools/tests/msgmerge-format-1: Renamed from gettext-tools/tests/msgmerge-13.
* gettext-tools/tests/msgmerge-format-2: Renamed from gettext-tools/tests/msgmerge-21.
* gettext-tools/tests/msgmerge-format-3: Renamed from gettext-tools/tests/msgmerge-28.
* gettext-tools/tests/msgmerge-plural-1: Renamed from gettext-tools/tests/msgmerge-10.
* gettext-tools/tests/msgmerge-plural-2: Renamed from gettext-tools/tests/msgmerge-11.
* gettext-tools/tests/msgmerge-plural-3: Renamed from gettext-tools/tests/msgmerge-12.
* gettext-tools/tests/msgmerge-plural-4: Renamed from gettext-tools/tests/msgmerge-22.
* gettext-tools/tests/msgmerge-plural-5: Renamed from gettext-tools/tests/msgmerge-23.
* gettext-tools/tests/msgmerge-plural-6: Renamed from gettext-tools/tests/msgmerge-24.
* gettext-tools/tests/Makefile.am (TESTS): Update.
