dnspolicy: Update single cluster tests to use istio gateway class

[mikenairn]

What

The DNS and TLS policy integration tests currently create their own gateway class for testing purposes allowing us to just set an expected status on the gateway and assert things based on that status. Other tests, auth and rlp, do not do this and instead use istio and expect it to be running somewhere and update the gateway with the status expected after all the required pieces are deployed (gateway, httproute, deployment etc..).
We should at least have some tests that do the same with DNS and TLS, multi cluster (OCM) tests won't be able to do this since they require the status to be in a specific format only produced by MGC currently.

Pros:

• More realistic, actually testing what will happen with a real gateway

Cons:

• A lot slower, requires the deployment of everything (gatway, httproute, deployment etc..) to get to a point of having something to test