Use ErrnoCheckingTest harness added in
d039af33096c0a83b03475a240d5e281e2271c44 for all unistd tests that
verify errno values. Stop explicitly setting it to zero in test code, as
harness does it.

It also verifies that the errno is zero at the end of each test case, so
update the ASSERT_ERRNO_EQ and ASSERT_ERRNO_FAILURE macro to clear out
its value after the verification (similar to how ErrnoSetterMatcher
does).

Update the CMake and Bazel rules for those tests. In the latter case,
remove commented out tests that are currently unsupported in Bazel,
since they get stale quickly.

Much of unistd involves modifying files. The tests for these functions
need to use libc_make_test_file_path which didn't exist when they were
first implemented. This patch adds most of unistd to the bazel along
with the corresponding tests. Tests that modify directories had to be
disabled since bazel doesn't seem to handle them properly.

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`.

Fixes #75261

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

Along the way, couple of additional things have been done:

1. Move `ErrnoSetterMatcher.h` to `test/UnitTest` as all other matchers live
   there now.
2. `ErrnoSetterMatcher` ignores matching `errno` on GPUs.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D151129

Differential Revision: https://reviews.llvm.org/D148454

This part of the effort to make all test related pieces into the `test`
directory. This helps is excluding test related pieces in a straight
forward manner if LLVM_INCLUDE_TESTS is OFF. Future patches will also move
the MPFR wrapper and testutils into the 'test' directory.