[libc++][test] Fix MSVC warning C4127 in `array.cons/initialization.pass.cpp` (#79793)

This fixes MSVC warning C4127: conditional expression is constant.

Testing `TEST_STD_AT_LEAST_20_OR_RUNTIME_EVALUATED` by itself doesn't
emit this warning, but the condition here is more complicated. I'm
expanding the macro and mechanically simplifying the resulting code.

(Yeah, this warning is often annoying, and I introduced
`TEST_STD_AT_LEAST_20_OR_RUNTIME_EVALUATED` to avoid this warning
elsewhere, so it's disappointing that it doesn't make the compiler happy
here. If this change is undesirable, I can replace it with
`ADDITIONAL_COMPILE_FLAGS(cl-style-warnings)`, but ideally I'd like to
avoid having to suppress it.)

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Co-authored-by: Louis Dionne <ldionne.2@gmail.com>