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:
 • f0053429
 • 1ec050ef

https://bugzilla.gnome.org/show_bug.cgi?id=648811