benchmark

Verify RegisterProfilerManager doesn't overwrite an existing registration (#1837)

* Verify RegisterProfilerManager doesn't overwrite an existing registration

Tested:
Add a second registration to test/profiler_manager_test.cc and
verify the test crashes as expected.

* Verify RegisterProfilerManager doesn't overwrite an existing registration

Tested:
Configure with:
cmake -GNinja -DCMAKE_BUILD_TYPE=Debug -DBENCHMARK_DOWNLOAD_DEPENDENCIES=on
Then run:
ctest -R profiler_manager_gtest
Before change test fails (expected), after change test passes (expected)

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Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>