tests: Add option --thread to tstgettext, tstngettext.

tests: Add option --thread to tstgettext, tstngettext.

* gettext-tools/tests/tstgettext.c: Include glthread/thread.h.
(inhibit_added_newline, do_expand): Remove variables.
(long_options): Add --thread option.
(struct worker_context): New type.
(main): Move some local variables into a context. Recognize option --thread.
At the end, invoke worker_thread.
(worker_thread): New function, extracted from main.
* gettext-tools/tests/tstngettext.c: Include glthread/thread.h.
(long_options): Add --thread option.
(struct worker_context): New type.
(main): Move some local variables into a context. Recognize option --thread.
At the end, invoke worker_thread.
(worker_thread): New function, extracted from main.
* gettext-tools/tests/Makefile.am (tstgettext_LDADD, tstngettext_LDADD): Add
gnulib-lib/libtestsgnu.a and $(LIBMULTITHREAD).