test: fix span_processor test failing with default features

[paullegranddc]

Some of the assertions are only passing if experimental_trace_batch_span_processor_with_async_runtime is enabled, which is not in the default features set, so running cargo test --workspace without specifying features is failing.

Changes

• Add a feature gate around failing assertions

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)

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: paullegranddc (914721e)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.7%. Comparing base (68c9133) to head (914721e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2782   +/-   ##
=====================================
  Coverage   79.7%   79.7%           
=====================================
  Files        123     123           
  Lines      23115   23117    +2     
=====================================
+ Hits       18426   18428    +2     
  Misses      4689    4689           

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