feat(gnoland): Add unit tests for Msg

[linhpn99]

This PR adds unit tests for gno.land/pkg/sdk/vm/msgs.go

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.47%. Comparing base (f04ec89) to head (a0da424).
Report is 25 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2726      +/-   ##
==========================================
+ Coverage   60.44%   60.47%   +0.03%     
==========================================
  Files         563      563              
  Lines       75159    75159              
==========================================
+ Hits        45427    45454      +27     
+ Misses      26343    26315      -28     
- Partials     3389     3390       +1     

Flag	Coverage Δ
contribs/gnodev	61.46% <ø> (ø)
contribs/gnofaucet	15.31% <ø> (ø)
gno.land	67.71% <ø> (+0.49%)	⬆️
gnovm	64.46% <ø> (ø)
misc/genstd	80.54% <ø> (ø)
misc/logos	20.23% <ø> (+0.35%)	⬆️
tm2	62.06% <ø> (+0.01%)	⬆️

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[leohhhn]

#2333 (comment)