Explicitly enable or disable conformant preprocessor in tests

[barcharcraz]

@CaseyCarter noted that in the near future, /std:c++latest is expected to imply /Zc:preprocessor, so we should alter all /std:c++latest configurations to explicitly enable or disable the new preprocessor.

Originally posted by @StephanTLavavej in #677 (comment)

Since /Zc:preprocessor is becoming the default, and we don't explicitly set it, we will see changes in test coverage once it is the default.

We should explicitly enable and disable it to get coverage of both modes.

[StephanTLavavej]

This issue is related to #435 (but not a duplicate).