[libc++] Avoid potential truncation warnings in std::abs test

One some platforms, -Wimplicit-int-conversion is enabled by default,
which can lead to additional warnings being triggered in this test.
Since we're only trying to test errors related to calling abs(), the
assignment is superfluous.

As a fly-by fix, correct one instance of ::abs to std::abs and made
the test a .verify.cpp test instead.

Differential Revision: https://reviews.llvm.org/D114244