Add initial tests to opentelemetry-appender-log

[mattbodd]

Fixes #1420

Changes

• Added tests to opentelemetry-appender-log crate to reach 100% of function coverage and 99%+ of line coverage via cargo-llvm-cov

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e3bee4e) 63.7% compared to head (a12b936) 63.7%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1501   +/-   ##
=====================================
  Coverage   63.7%   63.7%           
=====================================
  Files        144     144           
  Lines      19983   19983           
=====================================
  Hits       12741   12741           
  Misses      7242    7242           

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[mattbodd]

It doesn't look like the code coverage report reflects the new tests