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 patch removes some sched.h includes, preferring the proxy headers.
This patch does not clean them all up as some proxy type headers need to
be added, like for `struct sched_param`.

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

Previously used SYS_sched_get_priority_min. This also updates the tests
for SCHED_RR and SCHED_FIFO to check max_priority > min_priority, rather
than >= in order to catch future breakages.

Reviewed By: michaelrj, goldstein.w.n

Differential Revision: https://reviews.llvm.org/D149102

Includes macros:
    linux/SCHED_OTHER // posix req
    linux/SCHED_FIFO // posix req
    linux/SCHED_RR // posix req
    linux/SCHED_BATCH
    linux/SCHED_ISO
    linux/SCHED_IDLE
    linux/SCHED_DEADLINE

Includes types:
    struct sched_param { int sched_priority; }

Includes functions:
    sched_setparam
    sched_getparam
    sched_setscheduler
    sched_getscheduler
    sched_get_priority_max
    sched_get_priority_min
    sched_rr_get_interval

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D148069