selftests/damon: add a test for the pid leak of dbgfs_target_ids_write()

Commit ebb3f994 ("mm/damon/dbgfs: fix 'struct pid' leaks in
'dbgfs_target_ids_write()'") fixes a pid leak bug in DAMON debugfs
interface, namely dbgfs_target_ids_write() function.  Add a selftest for
the issue to prevent the problem from mistakenly recurring.

Link: https://lkml.kernel.org/r/20240207203134.69976-8-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>