add recipe for handling flaky tests

[emmettbutler]

What does this PR do?

This change adds brief and generic documentation about how to handle test unreliability

Motivation:

This change aims to make it easier for new contributors to become effective in this repo.

Change log entry

Additional Notes:

How to test the change?

[github-actions]

👋 Hey @DataDog/ruby-guild, please fill "Change log entry" section in the pull request description.

If changes need to be present in CHANGELOG.md you can state it this way

**Change log entry**

Yes. A brief summary to be placed into the CHANGELOG.md

(possible answers Yes/Yep/Yeah)

Or you can opt out like that

**Change log entry**

None.

(possible answers No/Nope/None)

^{Visited at: 2025-06-24 12:11:48 UTC}

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.55%. Comparing base (cd3ce6a) to head (e61304e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4752      +/-   ##
==========================================
- Coverage   97.56%   97.55%   -0.01%     
==========================================
  Files        1486     1486              
  Lines       88583    88583              
  Branches     4586     4586              
==========================================
- Hits        86425    86418       -7     
- Misses       2158     2165       +7     

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[datadog-datadog-prod-us1]

Datadog Report

Branch report: emmett.butler/flaky-guidance
Commit report: e61304e
Test service: dd-trace-rb

✅ 0 Failed, 21770 Passed, 1296 Skipped, 4m 45.15s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-06-20 21:04:53

Comparing candidate commit ad5fa4a in PR branch emmett.butler/flaky-guidance with baseline commit 2646e78 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 42 metrics, 5 unstable metrics.

[p-datadog]

We don't presently seem to run any tests for contributor PRs automatically (e.g. see: #4543). An employee has to push the code to this repo (which contribuotrs have no permission to do) for CI to run. I don't know if contributors will be able to restart any failing jobs either.

[TonyCTHsu]

@p-datadog , @emmettbutler is employed at Datadog. The Gitlab job failed without useful information only doc changes pull request.