Add API tests for subnet pool prefix operations

This introduces API tests for the subnetpool_prefix_ops
extension. These tests assert proper addition and removal of
prefixes against subnet pools under the various scenarios that
are supported.

Related-Bug: #1792901
Depends-On: https://review.opendev.org/#/c/648197/
Change-Id: I51564669fc1113556b0927296fa9dd2a8806bce8
3 files changed