Fix test failure on some PHP versions.

EntityRedirectResolvingDecoratorTest failed for some PHP version
with a rather obscure message related to serializing mock objects:

1) Wikibase\Lib\Test\Store\EntityRedirectResolvingDecoratorTest::testNoEntityId
Erroneous data format for unserializing 'Mock_EntityId_2c9fccb2'
/home/travis/build/wikimedia/phase3/extensions/Wikibase/lib/tests/phpunit/store/EntityRedirectResolvingDecoratorTest.php:76
...

See here:
https://travis-ci.org/wikimedia/mediawiki-extensions-Wikibase/jobs/29440080

Change-Id: I26dc3a6d133e7158165c877cb547c49373214493
Merge "Fix test failure on some PHP versions."