tests/connection-test: Ensure quit source id is reset to 0 between runs

Otherwise we may attempt to remove a nonexistent quit loop from the
previous one.  The second hunk here is the real fix, the first is just
being more defensive/consistent.

Reported-By: Kjartan Maraas <kmaraas@gnome.org>
Tested-By: Kjartan Maraas <kmaraas@gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=711361