bridge: Remove incorrect unthread-safe deprecated test

This was already broken, running channels in a thread is not
something we can do. The way that idle handlers, for example
are added to the wrong GMainContext is just the beginning of
issues here.

This code is deprecated so just remove the test.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
bridge: Simpler handling of closing channels during open

When opening a new channel, we need to wait until after the
mainloop has run before emitting signals, so that signal handlers
can be connected.

Previously each channel would do this in its own way. This puts
that in the base class so it's consistent.

Closes #1421
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>