kunit: allow kunit to be loaded as a module

Making kunit itself buildable as a module allows for "always-on"
kunit configuration; specifying CONFIG_KUNIT=m means the module
is built but only used when loaded.  Kunit test modules will load
kunit.ko as an implicit dependency, so simply running
"modprobe my-kunit-tests" will load the tests along with the kunit
module and run them.

Co-developed-by: Knut Omang <knut.omang@oracle.com>
Signed-off-by: Knut Omang <knut.omang@oracle.com>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>