tests: Add a function for asserting personas are in an aggregator
tests: Add a function for asserting personas are in an aggregator

This will be useful for testing the behaviour of individual
PersonaStores.

https://bugzilla.gnome.org/show_bug.cgi?id=712274

GNOME/folks,pwithnall/folks,pwithnall/folks,GNOME/folks,pwithnall/folks,GNOME/folks