eds: rework tests so that they don't depend on relative URIs

Previously, tests were build around the assumption that each
Edsf.PersonaStore had an id == ESource.relative_uri. That
stopped being true (because you'll get swallowed by a black
hole in pwithnall's garden if you use relative URIs).

Helps: https://bugzilla.gnome.org/show_bug.cgi?id=659732