Generalise the test helper 'genUniquesList'

Instead of always using "arbitrary", make it require a generator (that
can then use more complex rules for building the list elements), and
also convert it to use a temporary set instead of list membership.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>