doc: add guidance on parallel tests

[noahdietz]

We should prefer to run tests in parallel when they can be.

Based on #3462 (comment).

[gemini-code-assist]

Summary of Changes

Hello @noahdietz, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the Go style guide by adding a new section that encourages and provides instructions for running tests in parallel. It aims to improve testing efficiency by guiding developers on how to use t.Parallel() effectively, while also clearly defining the types of tests that should not be parallelized to avoid race conditions or incorrect behavior.

Highlights

• Parallel Test Guidance: Added a new section to the Go style guide (doc/howwewritego.md) providing explicit guidance on configuring tests to run in parallel using t.Parallel().
• Parallel Test Limitations: Included important caveats for parallel testing, specifically noting that tests mutating global state or using t.Chdir() are not suitable for parallel execution.

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[gemini-code-assist]

Code Review

This pull request adds valuable guidance on running Go tests in parallel to howwewritego.md. My review focuses on enhancing this new section to be more comprehensive by including a code example that demonstrates a common pattern and highlights a critical pitfall to avoid when writing parallel table-driven tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.06%. Comparing base (99bff58) to head (2d77f3d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3476   +/-   ##
=======================================
  Coverage   81.06%   81.06%           
=======================================
  Files         167      167           
  Lines       13823    13823           
=======================================
  Hits        11206    11206           
  Misses       2034     2034           
  Partials      583      583           

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[julieqiu]

Love it!