Fixes composer dependencies: Package phpunit/phpunit-mock-objects is abandoned
We cannot support v8 yet because of errors like:
Declaration of MailFunc::setUp() must be compatible with PHPUnit\Framework\TestCase::setUp(): void
It would require dropping PHP < 7.1 support.
Removed rcube_result_index::intersect() method.
Changed rcube_result_index::filter() to filter in the same way as
rcube_result_thread::filter(), which means it actually does array_intersect().
Added tests scripts for rcube_result_index class.
