[Serverless] Fix flaky tests

[maxday]

What does this PR do?

Tests related to Daemon/Lifecycle have been marked as flaky by datadog-ci
Those were not failing the CI as we retry 5 times each tests but this PR should fix them once for all :)

The flakiness came from:

1. fixed port in tests can have unpredictable behaviour in CI where a lot of tests are running, we may not control which port has been already taken or not
2. since ListenAndServe is started in its own go routine (see : https://github.com/DataDog/datadog-agent/blob/main/pkg/serverless/daemon/daemon.go#L126), it's possible that StartDaemon returns before actually being able to server /lambda/* endpoints

This problem is not actually an issue outside of tests since we start the daemon way before starting accepting invocation but it might be good to think about a more robust approach in the future

Additional Notes

Failure rate was quite high (more than 10%) and I was unable to reproduce a failure with this fix after running the tests 200 times.

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[nhinsch]

🙏 Thanks for fixing!