test suite: Add missing dependency on liburl-perl

test suite: Add missing dependency on liburl-perl

This is only a Recommends in dgit.deb so we need to pull it in
explicitly for the tests that need it.

We add the dependency unconditionally to the test suite tests that
depend on dgit, since that avoids having to track exactly when it's
needed.  (This module has an Installed-Size of 228.)

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>