test: suppress depreated warnings

    test-cut-pipeline.c: In function 'cut_startup':
    test-cut-pipeline.c:39:5: warning: implicit declaration of function 'g_mutex_new' [-Wimplicit-function-declaration]
         fixture_mutex = g_mutex_new();
         ^
    test-cut-pipeline.c:39:19: warning: assignment makes pointer from integer without a cast [enabled by default]
         fixture_mutex = g_mutex_new();
                       ^
    test-cut-pipeline.c:40:15: warning: assignment makes pointer from integer without a cast [enabled by default]
         run_mutex = g_mutex_new();
                   ^
    test-cut-pipeline.c: In function 'cut_shutdown':
    test-cut-pipeline.c:46:5: warning: implicit declaration of function 'g_mutex_free' [-Wimplicit-function-declaration]
         g_mutex_free(run_mutex);
         ^