selftests/damon: test no-op commit broke DAMON status

selftests/damon: test no-op commit broke DAMON status

Add test to verify that DAMON status is not changed after a no-op commit.

[ekffu200098@gmail.com: change wrong json.dump usage to json.dumps]
Link: https://lkml.kernel.org/r/20250816014033.190451-1-ekffu200098@gmail.com
Link: https://lkml.kernel.org/r/20250810124354.16456-1-ekffu200098@gmail.com
Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: Honggyu Kim <honggyu.kim@sk.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>