docker: Move dkid unittests to docker directory.

diff --git a/bacula/src/plugins/fd/dkid_test.c b/bacula/src/plugins/fd/docker/dkid_test.c
similarity index 100%
rename from bacula/src/plugins/fd/dkid_test.c
rename to bacula/src/plugins/fd/docker/dkid_test.c