ddtrace/tracer: only listen on localhost in TestTransportResponse

[nsrip-dd]

What does this PR do?

Listen on localhost in TestTransportResponse

Motivation

If we pass net.Listen an empty host, we'll listen on all available
addresses. macOS will warn about the app accepting incoming network
connections if we do this, creating an annoying popup each time we run
the whole tracer test suite. Using localhost is sufficient for this
test.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-12-21 14:46:59

Comparing candidate commit 7d19826 in PR branch nick.ripley/tracer-test-localhost-only with baseline commit a5129ef in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 39 metrics, 2 unstable metrics.

[darccio]

Yeah, good catch!