[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
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