Fix race condition where wrong caps were sometimes received before
the last gap internal clip by setting explicit framerate in video
caps and adjusting test timing accordingly.
The test now uses framerate=1/1 with single buffer expectations
instead of relying on default 30fps timing that created timing
races with the internal gap clip processing.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9608>
321601f069