test: video_output: add non-regression test

This commit introduces an integration test infrastructure which mocks
the external components (filters, decoders, window, display) used by the
video pipeline and starts this pipeline using the usual input item
properties and player.

The test will spawn an interface to have an initial vlc_object_t without
going though the private layers of libvlc (avoiding casting the instance
to private type, etc), so as to avoid encouraging code using such casts.

It uses this infrastructure to prevent regression from the ticket #26345.
By ensuring that the second failed decoder update won't crash, we can
check that an already started vout won't lead to double-join/free like
it was fixed in a7c99d05.

The test itself will later be extended to add more sensible cases.