phpunit static called method in method

I have this method:

public function getLocale()
    {
        $languageId = $this->_user->language->id;
        $page = Wire::getModule('myModule')->getPage($languageId);
        $locale = $page->locale;

        if (!!$locale) {
            return $locale;
        }

        // fallback to browser's locale
        $browserLocale = new SBD_Language();
        $locale = $browserLocale->getLanguageLocale('_');

        return $locale;
    }

Now I want to write a test for it, but I get this error: Trying to get property of non-object which is caused by Wire::getModule('myModule').

So I'd like to override the Wire::getModule response with phpunit. I just don't know how to do that.

So far I have created a mock over the class in which the method getLocale is placed and that is all working fine but how would I tell that mock class that it should actually call a mock of the Wire class?