contrib/miekg/dns: use random port for test

[rarguelloF]

What does this PR do?

Attempt to fix flaky test by using a random port instead of 2020.

Failed

=== RUN   TestExchangeContext
    dns_test.go:47: listen udp 127.0.0.1:2020: bind: address already in use
    dns_test.go:85: 
        	Error Trace:	/home/runner/work/dd-trace-go/dd-trace-go/contrib/miekg/dns/dns_test.go:85
        	Error:      	Received unexpected error:
        	            	read udp 127.0.0.1:45449->127.0.0.1:2020: read: connection refused
        	Test:       	TestExchangeContext
    dns_test.go:53: 
        	Error Trace:	/home/runner/work/dd-trace-go/dd-trace-go/contrib/miekg/dns/dns_test.go:53
        	            				/home/runner/work/dd-trace-go/dd-trace-go/contrib/miekg/dns/dns_test.go:90
        	Error:      	Received unexpected error:
        	            	dns: server not started
        	Test:       	TestExchangeContext
--- FAIL: TestExchangeContext (10.05s)

In addition, it changes the way the local server is started. Took inspiration on how it is done in the original library.

Ran the tests in this packages a few times with count=1000 and observed no flakes so far.

Motivation

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.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.
• For internal contributors, a matching PR should be created to the v2-dev branch and reviewed by @DataDog/apm-go.

Unsure? Have a question? Request a review!

[datadog-datadog-prod-us1]

Datadog Report

Branch report: rarguelloF/APMLP-378/dns-flaky-test
Commit report: c5db0b6
Test service: dd-trace-go

✅ 0 Failed, 5226 Passed, 73 Skipped, 1m 48.05s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-02-07 16:47:50

Comparing candidate commit f2ae871 in PR branch rarguelloF/APMLP-378/dns-flaky-test with baseline commit f11947f in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 62 metrics, 0 unstable metrics.

[hannahkm]

LGTM! Tested on Mac and Windows and it looks good. Thank you! 🙏🏼