Minor, move diagnostic tests for class objects

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

@@ -0,0 +1,31 @@
+
+package Jet86
+
+class A {
+  class object {
+    val x = 1
+  }
+  <!MANY_CLASS_OBJECTS!>class object { // error
+    val x = 1
+  }<!>
+}
+
+class B() {
+  val x = 12
+}
+
+object b {
+  <!CLASS_OBJECT_NOT_ALLOWED!>class object {
+    val x = 1
+  }<!> // error
+}
+
+val a = A.x
+val c = B.<!UNRESOLVED_REFERENCE!>x<!>
+val d = b.<!UNRESOLVED_REFERENCE!>x<!>
+
+val s = <!NO_CLASS_OBJECT!>System<!>  // error
+fun test() {
+  System.out.println()
+  java.lang.System.out.println()
+}