Move inmemory exporter for logs to diff namespace

[cijothomas]

opentelemetry_sdk::testing::logs::{InMemoryLogExporter, InMemoryLogExporterBuilder};
to
opentelemetry_sdk::logs::{InMemoryLogExporter, InMemoryLogExporterBuilder};

This is already under a feature-flag "testing", so this avoids the unnecessary "testing" module name.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.8%. Comparing base (5935cca) to head (415bb38).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2538   +/-   ##
=====================================
  Coverage   77.8%   77.8%           
=====================================
  Files        123     123           
  Lines      23047   23047           
=====================================
  Hits       17931   17931           
  Misses      5116    5116           

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[lalitb]

Thanks, we need similar for other in-memory exporter too.