Rename test gettext-6 to intl-thread-2 and support more platforms.

Rename test gettext-6 to intl-thread-2 and support more platforms.

* gettext-tools/tests/intl-thread-2: Renamed from gettext-tools/tests/gettext-6.
Don't limit to fixed set of platforms.
* gettext-tools/tests/intl-thread-2-prg.c: Renamed from
gettext-tools/tests/gettext-6-prg.c. Expect locale names as arguments.
* gettext-tools/tests/intl-thread-2-1.po: Renamed from
gettext-tools/tests/gettext-6-2.po. Convert to UTF-8.
* gettext-tools/tests/intl-thread-2-2.po: Renamed from
gettext-tools/tests/gettext-6-1.po. Convert to UTF-8.
* gettext-tools/tests/Makefile.am: Update.