Add unit tests for RTSP feature

[LukasBommes]

Figure out how to mock and unit test the RTSP feature. Prior to that I cannot merge community contributions that make changes to RTSP timestamp extraction.

Some useful links:

• Live555 Media Server: http://www.live555.com/mediaServer/
• mediamtx: https://github.com/bluenviron/mediamtx (usage example in https://github.com/insight-platform/Fake-RTSP-Stream/blob/main/docker-compose.yml)