Improvements to SiteLinkTableTest and use SimpleSiteLink in SiteLinkLookup interface

The second part is needed in the same commit to make the SiteLinkTable tests not fail,
as SiteLinkTable before needed to construct SiteLinks, which it cannot do without getting
the Site from the db, and thus have the test fail if those sites where not in the db. 

Change-Id: Ibc4b001fc607a02558f631b5a3bad476f07adc9e