mm/damon: move kunit tests to tests/ subdirectory with _kunit suffix

mm/damon: move kunit tests to tests/ subdirectory with _kunit suffix

There was a discussion about better places for kunit test code[1] and test
file name suffix[2].  Folowwing the conclusion, move kunit tests for DAMON
to mm/damon/tests/ subdirectory and rename those.

[1] https://lore.kernel.org/CABVgOS=pUdWb6NDHszuwb1HYws4a1-b1UmN=i8U_ED7HbDT0mg@mail.gmail.com
[2] https://lore.kernel.org/CABVgOSmKwPq7JEpHfS6sbOwsR0B-DBDk_JP-ZD9s9ZizvpUjbQ@mail.gmail.com

Link: https://lkml.kernel.org/r/20240827030336.7930-9-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: David Gow <davidgow@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>