j2k: flatten test cases and testData directory structure

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

j2k/testData/fileOrElement/nullability/FieldComparedWithNull.kt

@@ -0,0 +1,10 @@
+// ERROR: Unresolved reference: x
+class C {
+    private val s = x()
+
+    fun foo() {
+        if (s == null) {
+            System.out.print("null")
+        }
+    }
+}