Otherwise if there is a huge gap it will only be considered a
discontinuity after another discont-time amount of buffers has passed.
Like this it will be immediately a discontinuity if the gap between the
expected and received time becomes bigger than the discont-time.
The last part of the test was actually testing for this behaviour and
expected the previous behaviour. Most other tests also had to be
adjusted because discont will now happen at slightly different times
than before.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5759>