selftests/damon: test non-context inputs to rm_contexts file

There was a bug[1] that triggered by writing non-context DAMON debugfs
file names to the 'rm_contexts' DAMON debugfs file.  Add a selftest for
the bug to avoid it happen again.

[1] https://lore.kernel.org/damon/000000000000ede3ac05ec4abf8e@google.com/

Link: https://lkml.kernel.org/r/20221107165001.5717-3-sj@kernel.org


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