[libc++][test] Avoid preprocessor directives in macro argument lists (#73440)

Found while running libc++'s test suite with MSVC's STL.

MSVC has a level 1 "warning C5101: use of preprocessor directive in
function-like macro argument list is undefined behavior". I don't know
why Clang doesn't complain about this.

There are some formatting tests which densely interleave preprocessor
directives within function-like macros, and they would need invasive
changes. For now, I'm just skipping those tests.

However, a few tests were only slightly affected, and I was able to add
a new test macro `TEST_IF_AIX` to make them portable.