gdbus-exit-on-close test: cover more possibilities

gdbus-exit-on-close test: cover more possibilities

We didn't previously test anything except the implicit default of TRUE.
Now we test implicit TRUE, explicit TRUE, explicit FALSE, and
disconnecting at the local end (which regressed while fixing Bug #651268).

Also avoid some questionable use of a main context, which fell foul of
Bug #658999 and caused this test to be disabled in master.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=662100
Bug-NB: NB#287088
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: David Zeuthen <davidz@redhat.com>