Move trace sdk tests from trace_test into trace package

[dashpole]

I would like to be able to use a private option in #6393 in tests, and decided to split this refactoring out into its own PR.

This moves the batch span processor benchmarks into benchmark_test.go, and replaces one instance of the tracetest.NewInMemoryExporter with a different test exporter implementation. It then moves most unit tests from trace_test to the main trace package.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.8%. Comparing base (38f4f39) to head (094ba27).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6400     +/-   ##
=======================================
- Coverage   81.9%   81.8%   -0.1%     
=======================================
  Files        283     283             
  Lines      24899   24899             
=======================================
- Hits       20395   20379     -16     
- Misses      4101    4117     +16     
  Partials     403     403             

see 5 files with indirect coverage changes