check: Set harness caps in unit test before pushing buffers

Otherwise there will be warnings about data flow before stream-start
event as the harness is only sending the events once caps are set.

gstreamer-check/src/harness.rs

@@ -824,6 +824,7 @@ mod tests {
         gst::init().unwrap();
 
         let mut h = Harness::new("identity");
+        h.set_src_caps_str("application/test");
         let buf = gst::Buffer::new();
         let buf = h.push_and_pull(buf);
         assert!(buf.is_some());