Add test arg to AsyncValue.guard

[utamori]

Related Issues

fixes #3291

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).

• I have updated the CHANGELOG.md of the relevant packages.
  Changelog files must be edited under the form:

  ## Unreleased fix/major/minor
  
  - Description of your change. (thanks to @yourGithubId)

• If this contains new features or behavior changes,
  I have updated the documentation to match those changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a0b8d21) 95.16% compared to head (b478065) 95.16%.

❗ Current head b478065 differs from pull request most recent head c2aa9e9. Consider uploading reports for the commit c2aa9e9 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3292   +/-   ##
=======================================
  Coverage   95.16%   95.16%           
=======================================
  Files          53       53           
  Lines        2295     2298    +3     
=======================================
+ Hits         2184     2187    +3     
  Misses        111      111           

Files	Coverage Δ
packages/riverpod/lib/src/common.dart	100.00% <100.00%> (ø)

[rrousselGit]

Thanks for this!
Make sure to update the changelog and docs, as per the issue template :)

[rrousselGit]

You've edited the code-snippet. Explanation should be outside of the ```

Also, could you expand on it a bit more?

[utamori]

Thanks, I added it.

[rrousselGit]

This is a new feature. Update the heading to Unreleased minor

[rrousselGit]

LGTM, thanks!