selftests/damon: test DAMON enabling with empty target_ids case

DAMON debugfs didn't check empty targets when starting monitoring, and
the issue is fixed with commit b5ca3e83 ("mm/damon/dbgfs: add
adaptive_targets list check before enable monitor_on").  To avoid future
regression, this commit adds a test case for that in DAMON selftests.

Link: https://lkml.kernel.org/r/20211201150440.1088-9-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>