videotestsrc: keep track of the correct running time after renegotiations

videotestsrc: keep track of the correct running time after renegotiations

Need to store the old running time and frame numbers when renegotiating and
start from 0 again when a new caps is set, preventing that framerate changes
cause timestamping issues.

For example, if a stream pushed 10 buffers on framerate=2/1, its
running time will be 5s. If a new framerate of 1/1 is set, it would
make the running time go to 10s as it would count those 10 buffers
as being sent on this new framerate.

Fixes camerbin unit test.

https://bugzilla.gnome.org/show_bug.cgi?id=682973