Adds a new database connection info to Database.
protected function addConnection() {
// Add a new target to the connection, by cloning the current connection.
$connection_info = Database::getConnectionInfo($this->key);
Database::addConnectionInfo($this->key, $this->target, $connection_info[$this->originalTarget]);
// Verify that the new target exists.
$info = Database::getConnectionInfo($this->key);
// Note: Custom assertion message to not expose database credentials.
$this
->assertIdentical($info[$this->target], $connection_info[$this->key], 'New connection info found.');
}