Add Benchmark tests

Benchmarks are written as PHPUnit to make use of MwDBaseUnitTestCase and take advantage of the infrastructure such as to import data via XML or use a cloned DB.

Group `semantic-mediawiki-benchmark` is generally excluded from unit testing but doing `php mw-phpunit-runner.php --group semantic-mediawiki-benchmark` will allow to run benchmarks individually that are assigned to the `semantic-mediawiki-benchmark` group.

Relates to #510