The InterwikiLoadPrefix hook isn't compatible with Parsoid, as it is
unidirectional and doesn't support enumerating all valid prefixes
(T270444). Set/reset $wgInterwikiCache to mock the interwiki table
for parserTests and other unit tests instead.
This is a soft deprecation, as the used-in-production
Extension:Interwiki still uses InterwikiLoadPrefix, although not in a
way that would break Parsoid (since $wgInterwikiCache is set in
production).
Bug: T270444
Change-Id: If2507017c99c4ee42c104a0890bc45a84d7239d5
3d40c43b1f