Minor, move diagnostic tests for class objects

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

@@ -0,0 +1,22 @@
+package foo
+
+fun test() {
+  A.d
+  A.<!INVISIBLE_MEMBER!>f<!>
+  CCC.<!INVISIBLE_MEMBER!>classObjectVar<!>
+}
+
+class A() {
+  public class object {
+    val d = 3
+    private object f {
+
+    }
+  }
+}
+
+class CCC() {
+  private class object {
+    val classObjectVar = 3
+  }
+}