Refactor: process mentions of class object

Rename usages that refer to "default object" concept now
Test data file names are left as is

idea/testData/checker/ClassObjects.kt

@@ -4,7 +4,7 @@ class A {
   class object {
     val x = 1
   }
-  <error descr="[MANY_CLASS_OBJECTS] Only one class object is allowed per class">class object Another</error> { // error
+  <error descr="[MANY_DEFAULT_OBJECTS] Only one default object is allowed per class">class object Another</error> { // error
     val x = 1
   }
 }
@@ -14,7 +14,7 @@ class B() {
 }
 
 object b {
-  <error descr="[CLASS_OBJECT_NOT_ALLOWED] A class object is not allowed here">class object</error> {
+  <error descr="[DEFAULT_OBJECT_NOT_ALLOWED] A default object is not allowed here">class object</error> {
     val x = 1
   }
   // error