Reduce thread safety test output

[dlanov]

Fixes #145.

Summary

The thread-safety stress tests created plugin instances across 500 threads and called saySomething() for every instance, flooding the test output with repeated plugin messages.

Keep the shared-pointer and unique-pointer construction and destruction stress intact, but remove the unnecessary output-producing method calls.

Testing

Validated in ros:rolling-ros-base:

• colcon build --packages-select class_loader --cmake-args -DBUILD_TESTING=ON
• colcon test --packages-select class_loader
• 134 tests, 0 errors, 0 failures
• ClassLoaderTest.threadSafety passed
• ClassLoaderUniquePtrTest.threadSafety passed
• git diff --check

Generative AI

Generative AI assisted with issue analysis, preparing the scoped edit, and test planning. I reviewed the complete diff and all test results.

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