test: More slt tests for iszero function

[2010YOUY01]

Which issue does this PR close?

• Closes #.

Rationale for this change

With test coverage tool added in #23336, it's easy to identify files that are missing test coverage.

This PR adds test to one of them.

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.71%. Comparing base (67947b6) to head (d7a1f4e).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #23713      +/-   ##
==========================================
+ Coverage   80.67%   80.71%   +0.03%     
==========================================
  Files        1088     1088              
  Lines      367591   367741     +150     
  Branches   367591   367741     +150     
==========================================
+ Hits       296563   296817     +254     
+ Misses      53374    53211     -163     
- Partials    17654    17713      +59     

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[2010YOUY01]

main:

PR:

I'll try to figure out how to let the bot display such changes by default.