[libc++] [test] Use std::nextafter() instead of std::nexttoward()

Use std::nextafter() instead of std::nexttoward() in midpoint tests.
In the context of this test, this should not cause any difference.
Since nexttowardl() is not implemented on NetBSD 8, the latter function
combined with 'long double' type caused test failure.  nextafterl() does
not have this problem.

Differential Revision: https://reviews.llvm.org/D61748

llvm-svn: 360673