[ticket/12483] Allow functional and database tests to depend on (migrations of) extensions

https://tracker.phpbb.com/browse/PHPBB3-12483

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

It requires:
* **[.travis.yml](https://github.com/nickvergessen/phpbb-ext-acme-demo/blob/ef8359261100c81d6af06727d7a4f9b8416a6859/.travis.yml)** - To notify travis about the tests
* **[travis/prepare-phpbb.sh](https://github.com/nickvergessen/phpbb-ext-acme-demo/blob/ef8359261100c81d6af06727d7a4f9b8416a6859/travis/prepare-phpbb.sh)** - Moves the extension and clones phpbb
* **Adding the extension** to the [test file](https://github.com/nickvergessen/phpbb-ext-acme-demo/blob/ef8359261100c81d6af06727d7a4f9b8416a6859/tests/dbal/simple_test.php):
```
	static protected function setup_extensions()
	{
		return array('acme/demo');
	}
```