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.

It uses this infrastructure to check the behaviour in #26345 (closed).

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.

This requires to include the fix for #26345 (closed).