Add slogtest package for recording structured log output during tests

[maargenton]

• Add slogtest.Recorder as a recording implementation of slog.Handler.
• Add helper functions to extract all attributes from slog.Record.
• Add helper function to install temporary recorder as default slog logger.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.44%. Comparing base (da3d445) to head (d3c1072).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   93.13%   93.44%   +0.30%     
==========================================
  Files          29       30       +1     
  Lines        2055     2150      +95     
==========================================
+ Hits         1914     2009      +95     
  Misses        141      141              

Flag	Coverage Δ
unittests	93.44% <100.00%> (+0.30%)	⬆️

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[maargenton]

Released in v1.5.0