Cleanup WikibaseRepo tests and add covers tags

- improves code coverage reports for WikibaseRepo
- remove dependency on MediaWikiTestCase where not needed, which I think helps make tests run slightly faster.
- cleanup namespace usage
- miscellaneous cleanup

Change-Id: I519e0febeb8431d519edf3c80b92293112a1d421
Merge "Cleanup WikibaseRepo tests and add covers tags"