The page admin/config/development/testing should load if Selenium is in the file system but disabled.

Drupal displays all tests, even those pertaining to disabled modules in the file system. Therefore, if Selenium is disabled or not installed, Drupal will still attempt to display the tests on the admin/config/development/testing page, resulting in the error:
Fatal error: Class 'DrupalSeleniumWebTestCase' not found
There is little justification for having Selenium present in the filesystem without it being enabled, but when third-party module use Selenium-based tests in their test files, simply placing these modules in any Drupal site's file system causes the admin/config/development/testing to show an error. Consider, for example, #1436234: Fatal error when running simpletests if not selenium module is installed..
If this problem can be solved with the tests in Selenium itself, many developers will probably mimic the same structure in their own modules.