tests: Use GPG_ERROR_MT_LIBS for a test with threads.

* tests/Makefile.am (run_threaded_CPPFLAGS): New.
(run_threaded_LDADD): Replace GPG_ERROR_LIBS to GPG_ERROR_MT_LIBS.

• Reported-by: Ben Kibbey <bjk@luxsci.net>
• Fixes-commit: d0402f886b720d03369887c807581bd980ee70cf
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>