chore(sdk): remove tokio runtime from testing feature

[CathalMullan]

Related to #2643 I suppose.

Changes

The testing SDK feature doesn't actually need a tokio runtime, it's runtime agnostic, so removed it.

The opentelemetry-appender-log change might look irrelevant, but it's needed since feature unification meant this was previously enabled transitively.

This does mean the naming of the testing types is now a little misleading. They could just be called e.g. TestSpanExporter instead of TokioSpanExporter. Can rename them here if wanted.

EDIT: Actually, since the equivalent metrics type is called TestMetricReader, I'll go ahead with the rename for consistency.

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.
✅ Project coverage is 83.1%. Comparing base (c52f4a3) to head (71e947e).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3407     +/-   ##
=======================================
- Coverage   83.1%   83.1%   -0.1%     
=======================================
  Files        128     128             
  Lines      24877   24877             
=======================================
- Hits       20688   20684      -4     
- Misses      4189    4193      +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[bantonsson]

Thanks for the cleanup @CathalMullan, only one comment about the change log.