fix(tests): use global tokio runtime for tests [NR-520774]

[sigilioso]

Summary

This PR introduces a global helper to initialize the tokio runtime on unit tests. Additionally, it updates the HTTP Status Server shutdown: now it tries to stop both the server and the event-consumer even if the first stop operation fails.

Notes for reviewers

The #[serial] macro wasn't actually needed in the HTTP Status Server tests (it has been removed in this PR). When removing serial the test became more flaky and the issue was easy to reproduce using:

❯ for i in {1..15}; do echo "=== Run $i ===" && cargo test --package newrelic_agent_control --lib http_server::runner::tests 2>&1 || break; done

We already had a global helper for integration tests although it wasn't used everywhere (the corresponding integration tests are also updated in this PR). Considering the tokio runtime documentation and the fact of that we use a single runtime in Agent Control code, it seems the right path to follow.

[sigilioso]

There is something else 😓

I'll look into it!

[sigilioso]

Reproducible locally with enough repetitions

❯ for i in {1..50}; do echo "=== Run $i ===" && cargo test --package newrelic_agent_control --lib -- --skip on_host 2>&1 || break; done