tests: videodecoder: check that segment events are not dropped
Adds a test that simulates a scenario where the first buffers after a segment can't be decoded and the decoder asks for those frames to be released. The videodecoder base class should make sure that the events attached to those first buffers are pushed even if the buffers aren't going to be. https://bugzilla.gnome.org/show_bug.cgi?id=721835