objectcache: Fix HashBagOStuffTest test in PHP 7.1

objectcache: Fix HashBagOStuffTest test in PHP 7.1

> There was 1 error:
>
> 1) HashBagOStuffTest::testEvictionAdd
> A non-numeric value encountered
>
> tests/phpunit/includes/libs/objectcache/HashBagOStuffTest.php:106

This warning is new in PHP 7.1, however the behaviour is not new.
This code was already not behaving as it should have. Concatenation
preceeds addition/multiplication. As such, this was producing
(int)-10 each time instead of (str)"key0" through (str)"key10".

Change-Id: Ibb1a59e373740772f02dfec77ee7ebd9d181d852