j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/nullability/MethodResultComparedWithNull3.java

@@ -0,0 +1,13 @@
+//file
+interface I {
+    String getString();
+}
+
+class C {
+    void foo(I i) {
+        String result = i.getString();
+        if (result != null) {
+            print(result);
+        }
+    }
+}