Minor, move diagnostic tests for class objects

compiler/testData/diagnostics/tests/classObjects/ClassObjectCannotAccessClassFields.kt

@@ -0,0 +1,9 @@
+// http://youtrack.jetbrains.net/issue/KT-20
+
+class A() {
+    val x = 1
+
+    class object {
+        val y = <!INACCESSIBLE_OUTER_CLASS_EXPRESSION!>x<!>
+    }
+}