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

[mgorny]

Summary

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.

[mclow.lists]

I'm ok with this change, but I'm wondering - what is the reason that NetBSD doesn't implement nextafter?

[mgorny]

It's nexttowardl() that's missing in nb8. I don't really know why, probably nobody needed it at the point (I guess because it's no different than nextafterl()), it was put as TODO and forgotten. I've added it to nb9 but the backport to nb8 is currently blocked on some ABI stability debate. Until that's resolved, and since it doesn't seem to make any difference here, the change makes it possible to make tests pass again.

libcxx/trunk/test/std/numerics/numeric.ops/numeric.ops.midpoint/midpoint.float.pass.cpp

//  From e to pi
     assert((fptest_close_pct(std::midpoint(maxV, maxV), maxV,   pct)));
     assert((fptest_close_pct(std::midpoint(minV, minV), minV,   pct)));
 
 //  Denormalized values
 //  TODO
 
 //  Check two values "close to each other"
     T d1 = 3.14;
-    T d0 = std::nexttoward(d1, T(2));
-    T d2 = std::nexttoward(d1, T(5));
+    T d0 = std::nextafter(d1, T(2));
+    T d2 = std::nextafter(d1, T(5));
     assert(d0 < d1);  // sanity checking
     assert(d1 < d2);  // sanity checking
 
 //  Since there's nothing in between, the midpoint has to be one or the other
     T res;
     res = std::midpoint(d0, d1);
     assert(res == d0 || res == d1);
     assert(d0 <= res);