CI: Enable linting for tests

[cce]

Summary

I noticed linting isn't enabled for tests in the indexer repo, like it is in go-algorand, and think it would help catch issues with tests if it were. Enabled linting for tests in this PR and fixed the lint warnings.

Test Plan

Existing tests should pass and introduce new assertions (like previously unchecked errors)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.39%. Comparing base (249016c) to head (cf7bbd7).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1632   +/-   ##
=======================================
  Coverage   69.39%   69.39%           
=======================================
  Files          37       37           
  Lines        7375     7375           
=======================================
  Hits         5118     5118           
  Misses       1839     1839           
  Partials      418      418           

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