Rename test gettext-7 to intl-thread-3 and support more platforms.
* gettext-tools/tests/intl-thread-3: Renamed from gettext-tools/tests/gettext-7.
Don't limit to fixed set of platforms.
* gettext-tools/tests/intl-thread-3-prg.c: Renamed from
gettext-tools/tests/gettext-7-prg.c. Expect locale names as arguments.
* gettext-tools/tests/intl-thread-3.po: Renamed from
gettext-tools/tests/gettext-7.po.
* gettext-tools/tests/Makefile.am: Update.
