lavfi/vsrc_testsrc: switch to activate.

lavfi/vsrc_testsrc: switch to activate.

Allow to set the EOF timestamp.

Also: doc/filters/testsrc*: specify the rounding of the duration option.

The changes in the ref files are right.

For filter-fps-down, the graph is testsrc2=r=7:d=3.5,fps=3.
3.5=24.5/7, so the EOF of testsrc2 will have PTS 25/7.
25/7=(10+5/7)/3, so the EOF PTS for fps should be 11/7,
and the output should contain a frame at PTS 10.

For filter-fps-up, the graph is testsrc2=r=3:d=2,fps=7,
for filter-fps-up-round-down and filter-fps-up-round-up
it is the same with explicit rounding options.
But there is no rounding: testsrc2 produces exactly 6 frames
and 2 seconds, fps converts it into exactly 14 frames.

The tests should probably be adjusted to restore them to
a useful coverage.