Draft: net/webrtc: end 2 end and encoded filter tests

This test provides simple end 2 end 2 and encoded filter tests. They were designed to ensure non-regression while introducing the request-encoded-filter signals and can conveniently be used in a loop to detect race conditions.

Depends on

• ci: missing dtls elements in Windows images: #346 gstreamer-rs!1264 (merged)

TODO

• Fix a segmentation fault shutting down webrtcsink. The segmentation appears after the test has returned. Possibly fixed by !1221 (merged) & !1225 (merged).
• Fix a race condition leading to "Got data flow before segment event". Fixed by gstreamer!4844 (merged)
• Fix a race condition leading to gst_pad_create_stream_id_internal: assertion 'parent->numsinkpads <= 1' failed. Fixed by gstreamer!4844 (merged)
• Fix a deadlock handling ICE.
• Use gst_check::Harness where applicable.