Fix subnet mocking, related to #2510

Subnet mocking would return a set of all the current subnets on create (incorrectly) and thus when .first is called, the wrong subnet would be return for all but the first subnet created.

As a result, this left ``self.data`` in a bad state, and would cause the Network ACL tests to fail randomly depending on test ordering.

Thanks to @fcheung for finding a test that would cause this failure consistently. 

@reemus