Because Druplex is a mad-scientist hack, testing is complicated.
Basically, if you want to test this thing locally you must set up a Drupal 7 site in a directory called drupal/, and you should do it the way the .travis.yml file does, a bit like this:
$ drush make drushmake/fixture.make drupal
$ cp -r drushmake/fixture drupal/profiles
$ cp -r drushmake/druplex_feature drupal/sites/all/modules
$ cd drupal
$ drush si fixture --db-url=.....
Then you can finally go back to the root level and run PHPUnit:
$ cd ..
$ ./vendor/bin/phpunit
The tests are located in tests/ and they subclass the Silex/Symfony WebTestBase class, and use the Client class to mock requests through Request injection.