Change namespace of all Client Scribunto tests

To be honest I'm not exactly sure ...

1.) if this is the correct schema. It seems there are at least two,
    either "Wikibase\Client\<Module>\Test" or "Wikibase\Client\Tests\<Module>".
2.) if it's considered good practice to move tests in individual
    namespaces instead of a single "Tests" namespace for all of them.

Change-Id: I84d5b925a8ce1bad4348749599bda7d22071084e
Merge "Change namespace of all Client Scribunto tests"