j2k: flatten test cases and testData directory structure

Move j2k/test/tests -> j2k/tests, j2k/test/testData -> j2k/testData

j2k/testData/fileOrElement/function/mainAndNullabilitySetting.kt

@@ -0,0 +1,9 @@
+// !forceNotNullTypes: false
+public class A {
+    class object {
+        public fun main(args: Array<String>) {
+        }
+    }
+}
+
+fun main(args: Array<String>) = A.main(args)