Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated

compiler/testData/codegen/box/classes/classObjectInTrait.kt

@@ -0,0 +1,12 @@
+// EA-38323 - Illegal field modifiers in class: classObject field in C must be static and final 
+
+trait C {
+  class object {
+    public val FOO: String = "OK"
+  }
+}
+
+fun box(): String {
+  return C.FOO
+}
+