apparmor: add .kunitconfig

Add .kunitconfig file to the AppArmor directory to enable easy execution of
KUnit tests.

AppArmor tests (CONFIG_SECURITY_APPARMOR_KUNIT_TEST) depend on
CONFIG_SECURITY_APPARMOR which also depends on CONFIG_SECURITY and
CONFIG_NET. Without explicitly enabling these configs in the .kunitconfig,
developers will need to specify config manually.

With the .kunitconfig, developers can run the tests:
  $ ./tools/testing/kunit/kunit.py run --kunitconfig security/apparmor

Signed-off-by: Ryota Sakamoto <sakamo.ryota@gmail.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>