selftests/damon/sysfs.py: generalize DAMON context commit assertion

DAMON context commitment assertion is hard-coded for a specific test case.
Split it out into a general version that can be reused for different test
cases.

Link: https://lkml.kernel.org/r/20250720171652.92309-21-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>