Increasing code coverage in sdk/signing

[batconjurer]

Describe your changes

Increase code coverage in crats/core/signing.rs

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies:

[codecov]

Codecov Report

Attention: Patch coverage is 96.81934% with 25 lines in your changes missing coverage. Please review.

Project coverage is 73.83%. Comparing base (81467ed) to head (befc9d8).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/sdk/src/signing.rs	97.31%	21 Missing ⚠️
crates/wallet/src/lib.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3924      +/-   ##
==========================================
+ Coverage   73.25%   73.83%   +0.58%     
==========================================
  Files         341      341              
  Lines      105334   106114     +780     
==========================================
+ Hits        77158    78350    +1192     
+ Misses      28176    27764     -412     

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