add more tests for ante sig

[sahith-narahari]

Description

Ref: #6428

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[codecov]

Codecov Report

Merging #6462 into aaronc/6213-ante-sig-verify will increase coverage by 0.04%.
The diff coverage is 55.55%.

@@                       Coverage Diff                       @@
##           aaronc/6213-ante-sig-verify    #6462      +/-   ##
===============================================================
+ Coverage                        55.77%   55.81%   +0.04%     
===============================================================
  Files                              466      466              
  Lines                            27849    27850       +1     
===============================================================
+ Hits                             15532    15544      +12     
+ Misses                           11207    11192      -15     
- Partials                          1110     1114       +4     

[aaronc]

Mostly looks good. Just a few small comments.