Allow fetching from a specific git tag.

[Sean1708]

This PR updates the CMake module in the README to allow a git tag to be specified when using the fetch method. This allows people to use a specific version of test-drive without having to resort to git submodules or vendoring.

[awvwgk]

Thanks for sharing. Looks good to me.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.02%. Comparing base (2127360) to head (417e62a).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
- Coverage   71.51%   69.02%   -2.50%     
==========================================
  Files           2        2              
  Lines         481      481              
  Branches      296      261      -35     
==========================================
- Hits          344      332      -12     
  Misses         23       23              
- Partials      114      126      +12     

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