otelzap: refactor Core tests

[pellared]

Use the new convenience of https://pkg.go.dev/go.opentelemetry.io/otel/log/logtest.

The tests should be more readable and the assertions are more precise.
They also give better assertion messages on failures (diff).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.4%. Comparing base (47c05d8) to head (1aee923).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7380     +/-   ##
=======================================
- Coverage   82.4%   82.4%   -0.1%     
=======================================
  Files        205     205             
  Lines      18085   18085             
=======================================
- Hits       14907   14904      -3     
- Misses      2741    2743      +2     
- Partials     437     438      +1     

see 1 file with indirect coverage changes

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[khushijain21]

This looks great!