selftests/damon: add _damon_sysfs.py to TEST_FILES

selftests/damon: add _damon_sysfs.py to TEST_FILES

When running selftests I encountered the following error message with
some damon tests:

 # Traceback (most recent call last):
 #   File "[...]/damon/./damos_quota.py", line 7, in <module>
 #     import _damon_sysfs
 # ModuleNotFoundError: No module named '_damon_sysfs'

Fix this by adding the _damon_sysfs.py file to TEST_FILES so that it
will be available when running the respective damon selftests.

Link: https://lkml.kernel.org/r/20241127-picks-visitor-7416685b-mheyne@amazon.de
Fixes: 306abb63a8ca ("selftests/damon: implement a python module for test-purpose DAMON sysfs controls")
Signed-off-by: Maximilian Heyne <mheyne@amazon.de>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>