This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

Namely, template tests have been added for the following functions:
ceil, copysign, fabs, fmax, fmin, floor, trunc, round.

Summary:
Having a consistent prefix makes selecting all of the llvm libc tests
easier on any platform that is also using the gtest framework.
This also modifies the TEST and TEST_F macros to enforce this change
moving forward.

Reviewers: sivachandra

Subscribers:

This is a continuation of the previous CL which did a similar change in
other tests. To elaborate a little about why we need this - under C++
compilation with headers not from LLVM libc, libraries like libc++ and
libstdc++ provide their own math.h which undefine macros like `isnan`
and provide the overloaded C++ isnan functions which return a boolean
value instead of an integer value returned by the isnan macro.

The name `nan` conflicts with the function `nan` defined declared in
math.h.

This switches all of the files in src/string, src/math, and
test/src/math from using relative paths (e.g. `#include “include/string.h”`)
to global paths (e.g. `#include <string.h>`) to make bringing up those
functions on other platforms, such as fuchsia, easier.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D91394

This eliminates UnitTest's dependency on FPUtil and hence prevents
non-math tests from depending indirectly on FPUtil. The patch
essentially moves some of the existing pieces into a library of its own.

Along the way, renamed add_math_unittest to add_fp_unittest.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D85486

Summary: Add implementations of fmax, fmaxf, and fmaxl.

Reviewers: sivachandra

Subscribers: mgorny, tschuett, libc-commits, ecnelises

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D84385