binderfs_test: switch from /dev to a unique per-test mountpoint

Unprivileged users will be able to create directories in there. The
unprivileged test for /dev wouldn't have worked on most systems.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200313152420.138777-2-christian.brauner@ubuntu.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>