sync_api: run concurrent with other tests.
There's nothing timing-related or window-system related here, so run it concurrent. Dodges an unrelated softpipe flake where the setup of the frontbuffer rendering to the window (since this test also fails to set PIGLIT_VISUAL_DOUBLE) produced an error from X11, and that results in piglit atexit handlers firing ending with a segfault in X11 during teardown.