[ticket/11608] add basic search functional test structure

PHPBB3-11608
[ticket/11608] Skip tests for unsupported search backends

PHPBB3-11608
[ticket/11608] complete search tests functionalities

PHPBB3-11608
[ticket/11608] Use assertions in test

PHPBB3-11608
[ticket/11608] Add more assetions in tests

PHPBB3-11608
[ticket/11608] Logout before searching
Keyword search in search functional tests should be
performed as guests rather than logged in as admin.

PHPBB3-11608
[ticket/11608] Mark sphinx test as incomplete
Sphinx search for the test board cannot be tested

PHPBB3-11608
[ticket/11608] Check for word highlights in search results
Search tests check for highlighted words in search results

PHPBB3-11608
[ticket/11608] split search tests into separate files

Tests for each search backend are into their own separate files.
These separate classes inherit from a common search test case class.

PHPBB3-11608
[ticket/11608] Use assertContainsLang to compare language strings

PHPBB3-11608
[ticket/11608] Use assertEquals for more accurate assertions

PHPBB3-11608