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>

lib/kunit/test.c

@@ -486,3 +486,16 @@ void kunit_cleanup(struct kunit *test)
 	}
 }
 EXPORT_SYMBOL_GPL(kunit_cleanup);
+
+static int __init kunit_init(void)
+{
+	return 0;
+}
+late_initcall(kunit_init);
+
+static void __exit kunit_exit(void)
+{
+}
+module_exit(kunit_exit);
+
+MODULE_LICENSE("GPL v2");