Fix very slow unit-test data generation in some cases

Learning, learning. It turns out that for objects with even just five
fields, generating them and filtering can be costly, especially when
the filtering is nested (as it was in this case, arbitrary1 →
arbitrary2 + filter on arbitrary1 → arbitrary plus filter on
aribitrary2).

Changing the code to generate directly the good values speeds things
up significantly. Additionally, we also generate correctly unique
lists of disk templates (again, this should be a set, not a list,
but…).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>