Move name resolution cancellation test into Docker

[scalablecory]

#33420 introduces tests:

• DnsGetHostAddresses_PostCancelledToken_Throws
• DnsGetHostEntry_PostCancelledToken_Throws

These tests fail under a stressed CI environment as they depend on a cancellation token triggering before a DNS response comes back. To fix this, these tests might be moved into a Docker container with a non-working DNS server that will never respond.

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[gfoidl]

Instead of

be moved into a Docker container with a non-working DNS server that will never respond

(which is quite some work), can we "flood" the dns-queue with lookup tasks and see if at least any of them will be canceled?
Something like gfoidl@2281e5e

Although "a non-working DNS server that will never respond" will be safer here in regard of the race, but there will be more moving parts that need to play together nicely.

[antonfirsov]

Sounds like something to address when together (or as a precondition of) fixing #63552. I think instead of expensive infrastructure refactoring we may just try serializing them as we did with many socket tests