j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/nullability/FieldComparedWithNull.java

@@ -0,0 +1,10 @@
+//file
+class C {
+    private String s = x();
+
+    void foo() {
+        if (s == null) {
+            System.out.print("null");
+        }
+    }
+}