selftests/damon: classify tests for functionalities and regressions

DAMON selftests can be classified into two categories: functionalities and
regressions.  Functionality tests are for checking if the function is
working as specified, while the regression tests are basically reproducers
of previously reported and fixed bugs.  The tests of the categories are
mixed in the selftests Makefile.  Separate those for easier understanding
of the types of tests.

Link: https://lkml.kernel.org/r/20240503180318.72798-6-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>