j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/nullability/FieldComparedWithNull3.java

@@ -0,0 +1,12 @@
+//file
+class C {
+    public String s = "";
+}
+
+class D {
+    void foo(C c) {
+        if (null == c.s) {
+            System.out.println("null");
+        }
+    }
+}