[9.x] Adds database query count test expectation helper

[timacdonald]

New testing helper. Useful when you want to ensure the code doesn't inadvertantly introduce new database queries.

public function testItWorksEfficiently()
{
    $this->expectsDatabaseQueryCount(5);

    // do stuff...

    // throws an exception on tearDown if more than 5 queries have run
    // since the expectation was created.
}