Fixing getMockForTrait reference to getObjectForTrait

The current 3.8 documentation references a `getMockForTrait` method that does not exist; the appropriate method is `getObjectForTrait`. This change fixes all references to the `getMockForTrait` in the 3.8 version.

- Only en, ja, zh_cn were changed as other languages did not have the appropriate translation for testing traits
- Other versions were not changed as they do not have references to testing traits