tests: Run the threading tests only on UNIX.

* tests/gpg/Makefile.am (tests_unix): Add 't-thread-keylist' and
't-thread-keylist-verify'.
(c_tests): Drop them here.

The tests use pthreads and we suspect problems with MinGW's wrappers.

• Signed-off-by: Justus Winter <justus@g10code.com>