Use a scoped class for the host resolver in CNAME uncloaking tests

[antonok-edm]

Followup as per brave/brave-core#8601 (comment):

  network::HostResolver resolver(inner_resolver.get(), net::NetLog::Get());

  brave::SetAdblockCnameHostResolverForTesting(&resolver);

you also need to unset it somehow, so future tests would not get affected, see for example ScopedBrowserLocale or something like that

good point! I've updated the test to reset it to `nullptr again at the end, as well as some comments emphasizing that it's necessary. In the long-term some scoped solution would be ideal so I'll file a followup for that.

The CNAME tests in browser/brave_shields/ad_block_service_browsertest.cc use brave::SetAdblockCnameHostResolverForTesting, which needs to be manually reset to a nullptr after each test that uses it. This is an extra level of cognitive overhead when writing test code, so it'd be better to use a similar "scoped" class that will automatically reset when its destructor is called.