Fix Title library breaking other unit tests

Apparently assigning to $wgHooks at the top of the file breaks every
other hook, somehow. So we need to set it in setUp(), but prevent the
interwiki prefix from being queried beforehand when the dataProvider is
run. Fun.

Change-Id: I8535942e3a0e47d9a5b2ea11c1b0b63c5acf2cc5