selftests/damon: add a test for DAMON sysfs interface

This commit adds a selftest for DAMON sysfs interface.  It tests the
functionality of 'nr' files and existence of files in each directory of
the hierarchy.

Link: https://lkml.kernel.org/r/20220228081314.5770-12-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Xin Hao <xhao@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>