Unset "router:external" in subnet tests

Since [1], Neutron will return also the subnets belonging to an external
network. By default, the class ``SubnetsSearchCriteriaTest`` will not
consider these networks, same as in the class
``NetworksSearchCriteriaTest``.

[1]https://review.opendev.org/q/topic:%22bug/2051831%22

Depends-On: https://review.opendev.org/c/openstack/neutron/+/948073

Closes-Bug: #2076328
Related-Bug: #2051831
Related-Bug: #2082883
Change-Id: Id7048793d75640694299f340762f079111c81565
1 file changed