With MR 7156, transceivers and transports are created earlier, but for sendrecv media we could get `not-linked` errors due to transportreceivebin not being connected to rtpbin yet when incoming data arrives. This condition wasn't being tested in elements_webrtcbin, but could be reproduced in the webrtcbidirectional example. This commit now also adds a test for this, so that this doesn't regress anymore. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7294>