Fix test migration crash waiting to happen due to inferred relations Avoid crashing when a relation definition which is already known (due to being inferred) gets explicitly added. As explained in a comment, this would deserve more thinking, but at least this fixes a test failure with computed relations.

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/share/pyshared/cubicweb/skeleton/debian/rules.tmpl)