BUGFIX: Failed to get proper output dir for test-server

When creating the directory where to put the files needed by the test-server CMake would fail because it tried to create the directory /../share/libwebsockets-test-server, which is not possible. This happens since the TARGET_FILE_DIR for the test-server is not fetched properly.

Also mentioned in this trac ticket:
http://libwebsockets.org/trac/libwebsockets/ticket/84