key-file: Add missing licencing header

https://bugzilla.gnome.org/show_bug.cgi?id=712274
build: Factor common test build rules into test.mk

Simplify the build system a little.

https://bugzilla.gnome.org/show_bug.cgi?id=712274
build: Rename AM_VALAFLAGS to TARGET_VALAFLAGS in configure.ac

Remembering to always append to AM_VALAFLAGS, rather than define it from
scratch, is hard and error-prone. Simpler and more consistent to define a
new variable, TARGET_VALAFLAGS, which must be manually included in every
AM_VALAFLAGS or target-specific *_VALAFLAGS declaration.

https://bugzilla.gnome.org/show_bug.cgi?id=712274
build: Move backend.mk inclusions to the top of the files

Make it more obvious, and also make it non-optional.

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