Fix test.m.wikipedia.org

Commits 081c82 & 29c66f, part of a larger cleaning up Varnish config
effort, broke test.m.wikipedia.org. This happened as part of making
srv193 another backend for mobile-frontend, which however meant that it
shared all other options with the other backends, including port 81
instead of 80.

Make a quick hackish fix to reinstate the separate definition of the
test_wikipedia (ex-"test") backend and push it down with
$cluster_options.

Change-Id: Id18c5baca85977e9179f50f6ded54710e139dd3a