Ensure that feature code paths are well-tested

Compilers and standard libraries are inconsistent about the values of feature macros and such. It would be good to have a way to definitely turn on certain code paths for test. See https://github.com/intel/cpp-std-extensions/pull/116 for an example of how this fails.

No files or test entry points are named. Begin by comparing the failure described in pull request #116 and tracing the feature-macro decisions it exercises. Define a repeatable way to force selected code paths during tests, then verify that it works across the compiler and standard-library combinations described by the issue.

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