selftests: damon: add config file

Building and running the subsuite 'damon' of kselftest, shows the
following issues:
selftests: damon: debugfs_attrs.sh
/sys/kernel/debug/damon not found

By creating a config file enabling DAMON fragments in the
selftests/damon/ directory the tests pass.

Link: https://lkml.kernel.org/r/20230412092854.3306197-1-anders.roxell@linaro.org
Fixes: b348eb7abd09 ("mm/damon: add user space selftests")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>