Test creating a user with arbitrary uid.
function testUserImport() {
  // User ID must be a number that is not in the database.
  $max_uid = db_query('SELECT MAX(uid) FROM {users}')
    ->fetchField();
  $test_uid = $max_uid + mt_rand(1000, 1000000);
  $test_name = $this
    ->randomName();
  // Create the base user, based on drupalCreateUser().
  $user = entity_create('user', array(
    'name' => $test_name,
    'uid' => $test_uid,
    'mail' => $test_name . '@example.com',
    'pass' => user_password(),
    'status' => 1,
  ));
  $user
    ->enforceIsNew();
  $user
    ->save();
  // Test if created user exists.
  $user_by_uid = user_load($test_uid);
  $this
    ->assertTrue($user_by_uid, 'Loading user by uid.');
  $user_by_name = user_load_by_name($test_name);
  $this
    ->assertTrue($user_by_name, 'Loading user by name.');
}