Allow functional and database tests to depend on (migrations of) extensions

This allows extensions to easily unit, database and functional test their code, reusing as much code from phpBB as possible:
https://github.com/nickvergessen/phpbb-ext-acme-demo/pull/10 (Solution also depends on #2375 )

It requires:

• .travis.yml - To notify travis about the tests
• travis/prepare-phpbb.sh - Moves the extension and clones phpbb
• Adding the extension to the test file:

  static protected function setup_extensions()

  {

  return array('acme/demo');

  }

/cc VSE