Test coverage for network v2 subnets_client

According to the tempest lib documentation [0]:

"When adding a new interface to the library we need to at a
minimum have unit test coverage. A proposed change to add
an interface to tempest/lib that doesn't have unit tests
shouldn't be accepted. Ideally these unit tests will provide
sufficient coverage to ensure a stable interface moving forward."

Thus test coverage should be provided for existing clients in
tempest.lib that lack unit tests. This patch adds test coverage
for network v2 subnets_client.

[0] https://docs.openstack.org/developer/tempest/library.html#testing

Partially Implements: blueprint tempest-lib-missing-test-coverage
Change-Id: I5eadecf3c45c92a2bc058d093123014f04a2b781
1 file changed