Add tests in which `g_object_run_dispose()` is called on the source or target
of a `GBinding`. After commit a4fa456e677246629e714d05b5de178691571b06,
the target test caused a failed assertion in `g_weak_ref_set()` that was not
found by the existing tests. Commit 94ba14d5424c9304342a601cb9eb0842e56d8f51
weakens the assertion to allow the test to succeed.
See https://gitlab.gnome.org/GNOME/glib/-/issues/2676