test(dns): allow cancelled DoH family query

[ryanfowler]

Summary

• allow the dual-family DoH test to observe one or two wire requests
• document that the resolver cancels the pending family after a positive result

Validation

• cargo test --locked --all-features --test network dns_over_https_udp_and_inspect_dns_cases -- --exact
• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all-features --lib --bins