Stub out entry points in LookupTorrentURLTestCase

Torrent URL lookup functions are registered via entry-points.

If the tests run in an environment where the production lookup function is
present, it will use that; not what we want for tests.

Instead, we should stubout entry-points so we have a mock URL lookup function.

Fixes bug 1211396

Change-Id: Ibc2b34bfdcecaad4c53fb00e925a8b21965d8577
Merge "Stub out entry points in LookupTorrentURLTestCase"