Change testwiki config to be closer to enwiki

Except for the presence of the VisualEditor namespace, testwiki now
behaves the same as enwiki. Changes:
* VE enabled in main and user namespaces, as well as the VisualEditor namespace
* VE no longer enabled by default
* VE preference no longer hidden

Change-Id: I4410f6be2212364137bc312c5b19155f9be85001