selftests/damon/sysfs_refresh: test kdamond refresh_ms

Writing a non-zero value to a kdamond's 'refresh_ms' sysfs file should
make DAMON periodically update the read-only sysfs files on its own,
without the user writing update keywords such as 'update_schemes_stats' to
the 'state' file.  This behavior has no test coverage.

Add a test that starts a kdamond with refresh_ms set and a 'stat' scheme
whose default access pattern matches every monitored region, then polls
the scheme's 'nr_tried' stats file directly, without requesting an update.
The value can become non-zero only via the periodic refresh, so the test
confirms refresh_ms works; with refresh_ms disabled the stat stays zero
and the test fails.

Link: https://lore.kernel.org/20260602131217.2210912-3-linuxoid@gmail.com
Link: https://lore.kernel.org/20260629144927.134237-3-sj@kernel.org
Signed-off-by: Ruslan Valiyev <linuxoid@gmail.com>
Reviewed-by: SJ Park <sj@kernel.org>
Signed-off-by: SJ Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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