Add integration test for `pulsevideo` and `dbusvideosourcesrc`
dbusvideosourcesrc: Fix locking setting the DBus connection
dbusvideosourcesrc: Remove unnecessary cast

The `bsrc` parameter to `gst_dbus_videosource_src_start` is left over from
when `dbusvideosourcesrc` was a `GstBaseSrc` and it no longer necessary
now that it's a `GstBin`.
Remove setting socketsrc blocksize

We now use additional payloading which preserves buffer edges in the form
of `fdpay`/`fddepay` so this is no-longer necessary.
`make check`: Fix running against local version of pulsevideo plugins
socketsrc: Add `on-socket-eos` signal

This provides notification that the socket in use has reached EOS and gives
and opportunity to replace it with a new one which is not EOS, allowing
reading from many sockets in order.
socketsrc: Tidy up usage of `g_object_unref`/`g_clear_object` and locking

No functional change intended.
dbusvideosourcesrc: Set socketsrc live

This is better than returning `NO_PREROLL` ourselves as the real source
of the pipeline (`socketsrc`) now knows that it's live and will behave
accordingly (based on whatever is coded into `GstBaseSrc`).
dbusvideosourcesrc: Attempt reconnection if socket is closed remotely

This allows `dbusvideosourcesrc` to recover from `pulsevideo` crashing
and gives a way to signal to clients that there has been a change in
caps, etc.