test: video_output: fix test not ending correctly

The test was failing randomly because the decoder was asked to decode
more data and its output format was reset to VLC_CODEC_UNKNOWN, which is
not an allocatable format for picture_NewFromResource.

Abandon the picture allocation in decoder_decoder(dec, picture) callback
to let the scenario handle the allocation, and in particular discard it
if it already did the test. The new test_finished boolean will force the
decoder callback to no-op. It is a bit redundant with the display_opened
callback where it could loop doing the update until it is correctly
closed but much clearer.

I'm not sure yet, but it seems that the easiest method to control the
behaviour of the test is to have a custom demux to push data at will.
It's adding a lot of code and is trickier to manage so here we fix the
test first.

Fixes #26374