Allow modules to be disabled from local.xml.phpunit

We have found that there are some third-party or intractable modules that apply settings or make assumptions about execution occurring in a CGI or mod_php context. This pull request allows them to be disabled during unit testing by extending the Mage_Core_Model_Config::_isAllowedModule() method. 

Uses the following XML structure with the module under test's config.xml:

    <phpunit>
        <disable_modules>
            <Mynamespace_Mymodule/>
        </disable_modules>
    </phpunit>