Add Go tests in MySQL storage implementation

[roger2hk]

#21

Note that this pull request also makes tessera.WithCheckpointSignerVerifier optional in mysql.New() because the issue was found during the tests development.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.62%. Comparing base (46ec9c2) to head (80aeb03).
Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #110      +/-   ##
==========================================
- Coverage   35.80%   28.62%   -7.18%     
==========================================
  Files          16       31      +15     
  Lines        1363     2683    +1320     
==========================================
+ Hits          488      768     +280     
- Misses        801     1828    +1027     
- Partials       74       87      +13     

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[roger2hk]

Would the 655536 be too large in the TestParallelAdd test?