Add multi-logged integration tests to otelslog

[MrAlias]

Follow up to #5314

Part of #5195

Copy the testing framework from slogtest and use it to test the otelslog.Handler functionality complexity not tested by the base slogtest package.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.8%. Comparing base (336d5b6) to head (3eeced9).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5329     +/-   ##
=======================================
+ Coverage   61.7%   61.8%   +0.1%     
=======================================
  Files        186     186             
  Lines      11388   11388             
=======================================
+ Hits        7032    7049     +17     
+ Misses      4153    4137     -16     
+ Partials     203     202      -1     

see 1 file with indirect coverage changes