video: Remove nonsensical test

Printing an unknown video format returns NULL, and with latest git main
this actually causes a critical warning in addition.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1430>

gstreamer-video/src/video_format.rs

@@ -480,13 +480,6 @@ mod tests {
         );
     }
 
-    #[test]
-    #[should_panic(expected = "gst_video_format_to_string returned NULL")]
-    fn enum_to_string_panics() {
-        assert_eq!(&format!("{}", crate::VideoFormat::__Unknown(-1)), "UNKNOWN");
-        assert_eq!(crate::VideoFormat::__Unknown(-1).to_str(), "UNKNOWN");
-    }
-
     #[test]
     fn test_display() {
         gst::init().unwrap();