j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/nullability/MethodInvokedWithNullArg2.java

@@ -0,0 +1,10 @@
+//file
+class C {
+    public void foo(String s){}
+}
+
+class D {
+    void bar(C c) {
+        c.foo(null);
+    }
+}