[libcxx] [test] Fix MSVC warning C4127 "conditional expression is constant".
MSVC has a compiler warning (enabled at /W4) that's potentially useful,
although slightly annoying to library devs who know what they're doing.
In the latest version of the compiler, the warning is suppressed when the
compiler sees simple tests like "if (name_of_thing)", so extracting comparison
expressions into named constants is a workaround. This is useful for ensuring
that libraries are C4127-clean.