vcap/testcard: Fix frames being limited to 0 by default

The capture_frames was initialized to 0 which together with changes in
commit 2d72943 meant that no frames were captured by default.

src/video_capture/testcard.c

@@ -524,6 +524,7 @@ static int vidcap_testcard_init(struct vidcap_params *params, void **state)
         }
         strncat(s->pattern, DEFAULT_PATTERN, sizeof s->pattern - 1);
         s->audio_frequency = DEFAULT_AUIDIO_FREQUENCY;
+        s->capture_frames = -1;
 
         char *fmt = strdup(vidcap_params_get_fmt(params));
         char *ptr = fmt;