Testing a module which uses CiviCRM php api
I'm not an expert of testing but i'm pretty sure that SimpleTest creates a new instance of Drupal with a new database each time the testing process is ran.

CiviCRM is not a standard Drupal module and cannot just be enabled. In your SetUp function, You need to install CiviCRM to create the corresponding tables in the database. It could be a rather complex task but it's possible to do so. There is a drush script to do it so it might be a good inspiration.

You might want to check drush_civicrm_install in https://github.com/civicrm/civicrm-drupal/blob/7.x-master/drush/civicrm.drush.inc

Otherwise, you might find easier to use CiviCRM testing way (phpUnit) : https://wiki.civicrm.org/confluence/display/CRMDOC/Testing

And an example : https://github.com/civicrm/civicrm-core/blob/master/tests/phpunit/api/v3/ActivityTest.php