j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/function/mainAndNullabilitySetting.java

@@ -0,0 +1,6 @@
+// !forceNotNullTypes: false
+//file
+public class A {
+    public static void main(String[] args) {
+    }
+}