Create a genInstanceMaybeBiggerThan function

This is used only once when testing Cluster.hs, but having it abstracted
clarifies there what that call is about, makes that test shorter, and
allows us to better do refactoring of the main genInstanceSmallerThan
generator.

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