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

Explicit braces were added to fix the "suggest explicit braces to avoid
ambiguous ‘else’" warning since the current solution (switch (0) case 0:
default:) doesn't work since gcc 7 (see
https://github.com/google/googletest/issues/1119)

gcc 13 generates about 5000 of these warnings when building libc without
this patch.

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, only those unit tests which belonged to a suite were run as
part of libc-unit-tests. It meant that unit tests not part of any suite
were not being tested. This change makes all unit tests run as part of
libc-unit-tests. The convenience function to add a libc unit test suite
has been removed and add_custom_target is used in its place. One of the
bit-rotting test has along been fixed. Math exhaustive and differential
tests are skipped under full build.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D148784

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.

Namely, posix_spawn_file_actions_addclose,
posix_spawn_file_actions_adddup2, posix_spawn_file_actions_addopen,
posix_spawn_file_actions_destroy, posix_spawn_file_actions_init have
been added.

Reviewed By: michaelrj, lntue

Differential Revision: https://reviews.llvm.org/D135603