Follow-up from "element.py: remove call to _source_cached()" -- test_workspace_visible changed to XFAIL

The following discussion from !1612 (merged) should be addressed:

key is not being set until after committing the workspace to the sourcecache (under the ref workspace/None) meaning that subsequent calls to Element._source_cached() will fail (since the key is not associated with the storage)

Edited by Darius Makovsky