Refactor tests

Hi there,

I've modified the unit tests to reduce the environment requirements.

  * No root privileges required.
  * Starting and stopping FTP server is out of scope: removed.
  * Accessing remote directory only via FTP, should work with any FTP server.
  * Overriding FTP account details via environment variables enabled.

Tested on Debian Wheezy with vsftpd on localhost with system user access.