j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/nullability/MethodResultComparedWithNull.java

@@ -0,0 +1,12 @@
+//file
+interface I {
+    String getString();
+}
+
+class C {
+    void foo(I i) {
+        if (i.getString() == null) {
+            println("null");
+        }
+    }
+}