tests: Add a standalone-individuals test program

This contains a few test cases which check folks’ behaviour when
manually creating Individuals *outside* an IndividualAggregator. These
should catch the problems found in the following two bugs:
 • https://bugzilla.redhat.com/show_bug.cgi?id=1031252
 • https://bugzilla.gnome.org/show_bug.cgi?id=712839
and hopefully fixed by commits:
 • f00534294d7d52ac7e37dfaa075e3465b7755483
 • 1ec050efc4f7135e9958c74da2028daf669077a0
individual: Add default values for PresenceDetails properties

If an Individual is created with no Personas (as in the
standalone-individuals test cases), the PresenceDetails properties
should be initialised to empty strings. They were previously defaulting
to null.
individual: Allow Individual.id to be an empty string with no personas

If an Individual has no Personas (as can happen when it’s just been
constructed), document the ID as being the empty string. This is a
special case.

Previously, the ID defaulted to being null, which violated its specified
type.