test: rework interwiki lookup in parser tests

Some of our parser tests lookup interwikis.  This was originally done
(parser/parserTest.inc) by inserting a set of interwikis in the database
and was later lamely copy pasted in the PHPUnit wrapped test suite
(phpunit/includes/parser/NewParserTest.php).

Since that time, we had duplicate code and had the test hitting the
database to fetch interwiki.  Nowadays, we can trick the Interwiki
lookup class by using the InterwikiLoadPrefix hook, that let us skip
database lookup entirely (by having the hook returning false) and get
rid of the duplicate code.

The good old parserTests.php still pass the tests :-]

Change-Id: I36865e3890e08a05b8a6aaafa309a87556e235b9