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

https://tracker.phpbb.com/browse/PHPBB3-11528

- [x] Refactor install_board() code into parent class phpbb_mink_test_case
- [x] Add mink and its drivers as dependency
- [x] Make phpbb_mink_test_case independent of mink driver and client
- [x] Add asserts to install_board( )
- [ ] Add UI test class
- [ ] Add some initial UI tests
- [ ] Run UI tests on travis
Can you try composer without dependencies so this is no bumped? mink should be happy with 2.3.4. Otherwise I would suggest to bump the whole symfony suite in a new PR (probably a good idea).