rtp: tests: print rtp timestamp mismatch minus the initial offset

Unit tests specify a 0-based offset, so printing that plus the
random initial offset on failure is just needlessly confusing,
so subtract the initial offset when printing expected/actual
values. The real values are still printed as part of the assert.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1580>

net/rtp/src/tests.rs

@@ -445,8 +445,8 @@ pub fn run_test_pipeline_full(
                 "Buffer {} of payload buffer list {} has unexpected RTP timestamp {:?} instead of {:?}",
                 j,
                 i,
-                rtp_packet.timestamp(),
-                expected_timestamp,
+                rtp_packet.timestamp().wrapping_sub(initial_timestamp),
+                expected_timestamp.wrapping_sub(initial_timestamp),
             );
         }
     }