[libc] Migrate dirent/fcntl/sched/signal tests to ErrnoCheckingTest. (#158700)

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

GitOrigin-RevId: 6512bf0e857c8926e3bb0a07c1eac270faf4ae3f
18 files changed