Remove direct libc_errno.h inclusion and manipulation of libc_errno in
various unit tests, instead relying on ErrnoCheckingTest machinery.

This is the final mechanical change of migrating libc unit tests to
ErrnoCheckingTest - after it, all the unit tests relying on
ASSERT_ERRNO_* macro will be using ErrnoCheckingTest.h

This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

Having libc_errno outside of the namespace causes versioning issues when
trying to link the tests against LLVM-libc. Most of this patch is just
moving libc_errno inside the namespace in tests. This isn't necessary in
the function implementations since those are already inside the
namespace.

This patch provides specific test macros to deal with `errno`.
This will help abstract away the differences between unit test and integration/hermetic tests in #79319.
In one case we use `libc_errno` which is a struct, in the other case we deal directly with `errno`.

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

Implements: https://linux.die.net/man/2/sched_yield

Possibly we don't need the return value check / errno as according to
both the manpage (and current linux source) `sched_yield` cannot fail.

Reviewed By: sivachandra, michaelrj

Differential Revision: https://reviews.llvm.org/D147985