Improve core classes unit testing
- Remove @covers for inexistent class. It was folling phpunit to believe
that no code coverage was provided for ReverseInterwikiLookup.
- Test getDeletedEntry for Utils.php.
- Increase delta for assertTimestampEquals.
Bug: T204270
Change-Id: Ib9eeb802cde6d9837e4395be2bc3239bd59acd2e