doc: add separate error test guidance

[noahdietz]

Add simple guidance for separating error tests from functional tests based on https://google.github.io/styleguide/go/decisions.html#table-driven-tests and http://go/gotip/episodes/50.

[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 enhances the Go style guide documentation by introducing a dedicated section on best practices for writing error tests. The changes aim to improve the clarity, maintainability, and robustness of test suites by advocating for the separation of error and functional tests, specific naming conventions, and proper error comparison techniques.

Highlights

• New Error Testing Guidance: Added a new section to the howwewritego.md document providing guidance on separating error tests from functional tests.
• Test Function Naming Convention: Recommended using the TestXxx_Error naming convention for error test functions to clearly distinguish them.
• Error Comparison Best Practice: Advised using errors.Is for comparing errors in tests, promoting more robust and idiomatic error handling.
• Code Example for Error Tests: Included a detailed Go code example demonstrating a table-driven error test using the recommended practices.

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[gemini-code-assist]

Code Review

This pull request adds documentation for writing separate error tests in Go. The new section is a good addition to the guide. However, the provided Go code example contains several syntax errors that would prevent it from compiling. I've provided a corrected version of the code snippet that fixes these errors and also aligns with the existing guidance in the document for using t.Run in table-driven tests.

[noahdietz]

/gemini review

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.05%. Comparing base (32f4200) to head (f761c41).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3475   +/-   ##
=======================================
  Coverage   81.05%   81.05%           
=======================================
  Files         166      166           
  Lines       13817    13823    +6     
=======================================
+ Hits        11199    11204    +5     
- Misses       2032     2035    +3     
+ Partials      586      584    -2     

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[gemini-code-assist]

Code Review

This pull request adds valuable guidance to howwewritego.md on separating error tests from functional tests. The explanation and example are clear and follow the linked Google style guide. I've suggested a few minor improvements to the code example for better formatting consistency and more informative test failure messages, as well as a small fix for a trailing space in the markdown.