[ticket/11528] Add behat/mink and mink-goutte-driver as dependency

PHPBB3-11528
[ticket/11528] Refactor install_board() code into a new class

PHPBB3-11528
[ticket/11528] Instantiate $client, $driver from functional test case

Make mink test case an abstract class, instantiate $driver, $client
from its child class i.e. functional test case.

PHPBB3-11528
[ticket/11528] Call parent::tearDown in overrided tearDown method

PHPBB3-11528
[ticket/11528] Move parent::tearDown() to end of tearDown()

PHPBB3-11528
[ticket/11528] Use 'use' keyword to import classes to current scope

PHPBB3-11528
[ticket/11528] Add asserts in installation steps

PHPBB3-11528
[ticket/11528] Fix spaces and slashes as per guidelines

PHPBB3-11528
[ticket/11528] Change NULL to null

PHPBB3-11528
[ticket/11528] Handle case when config file cant be written by installer

PHPBB3-11528
Merge pull request #2587 from dhruvgoel92/ticket/11528

[ticket/11528] Refactor install_board( ) into a new mink class

* dhruvgoel92/ticket/11528:
  [ticket/11528] Handle case when config file cant be written by installer
  [ticket/11528] Change NULL to null
  [ticket/11528] Fix spaces and slashes as per guidelines
  [ticket/11528] Add asserts in installation steps
  [ticket/11528] Use 'use' keyword to import classes to current scope
  [ticket/11528] Move parent::tearDown() to end of tearDown()
  [ticket/11528] Call parent::tearDown in overrided tearDown method
  [ticket/11528] Instantiate $client, $driver from functional test case
  [ticket/11528] Refactor install_board() code into a new class
  [ticket/11528] Add behat/mink and mink-goutte-driver as dependency