[TEST] Code cleanup in exporter tests

[marcalff]

Fixes # (issue)

Issue

Exporter unit tests contain:

#ifndef OPENTELEMETRY_STL_VERSION

This:

• may disable the tests in CI, depending of the setting for STL_VERSION
• may disable include-what-you-use, likewise
• may disable clang-tidy, likewise

Note that the iwyu build is using -DOTELCPP_WITH_STL=CXX14, so iwyu was effectively ignoring these tests.

Changes

Please provide a brief description of the changes here.

• Removed the test on OPENTELEMETRY_STL_VERSION.
  • This was an old work around for an issue in GMock. The work around is no longer necessary.
• Applied clang-format to the code
• Resolved all issues reported by include-what-you-use

This ensures that these unit tests are always built and executed.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.47%. Comparing base (8165c2d) to head (fdc7b3b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4158      +/-   ##
==========================================
+ Coverage   83.46%   83.47%   +0.01%     
==========================================
  Files         521      521              
  Lines       20380    20380              
==========================================
+ Hits        17009    17011       +2     
+ Misses       3371     3369       -2     

Files with missing lines	Coverage Δ
exporters/otlp/src/otlp_grpc_exporter_factory.cc	50.00% <ø> (ø)

... and 1 file with indirect coverage changes

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[marcalff]

@dbarker

Please review

[dbarker]

Looks good. Thanks.