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.

usr/lib/PY_VERSION/*-packages/cubicweb/web
usr/share/cubicweb/cubes/shared/data
usr/share/cubicweb/cubes/shared/wdoc