j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/nullability/FieldComparedWithNull4.java

@@ -0,0 +1,11 @@
+//file
+class C {
+    private String s;
+
+    public C(String s) {
+        this.s = s;
+        if (s == null) {
+            System.out.print("null");
+        }
+    }
+}