Non-hermetic DNS testing in eg dynamic_forward_proxy is causing recurring failure/slowness

I stumbled upon this issue with the mobile test `//test/common/integration:client_integration_test`

in that case the test had a total timeout and was querying dns for `doesnotexist` - dns was retrying 3x, and taking 5+ seconds to respond on each try - blowing the tests total timeout budget

im struggling to find how i resolved the issue in mobile - but it seems i got past it

this problem has again occurred when trying to update the build image - asan and gcc are reliably timeouting on similar tests - and it looks like for similar reasons

i would have thought these tests should not be calling any real dns - it should be mocked and hermetic