[libc++][test] Mark `test_comparisons.h` helpers as nodiscard

I accidentally wrote `testComparisons(...)` instead of
`assert(testComparisons(...))`. This compiled without issues, but
did not provide the intended test coverage. By adding a `nodiscard`,
we can make sure that the compiler catches such mistakes for us.

Differential Revision: https://reviews.llvm.org/D131364