selftests: add devpts selftests

This adds tests to check:
- bind-mounts from /dev/pts/ptmx to /dev/ptmx work
- non-standard mounts of devpts work
- bind-mounts of /dev/pts/ptmx to locations that do not resolve to a valid
  slave pty path under the originating devpts mount fail

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

tools/testing/selftests/filesystems/Makefile

@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0
-TEST_PROGS := dnotify_test
+TEST_PROGS := dnotify_test devpts_pts
 all: $(TEST_PROGS)
 
 include ../lib.mk