Add SimpleMultiQueryBenchmarkTest

Benchmarks are written as PHPUnit to make use of `MwDBaseUnitTestCase` and
take advantage of the infrastructure such as to import data via XML or
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.