Skip known-failing tests

Skip several tests for the System entity. In particular:

* Skip the "delete" test. When a non-existent system is fetched, an HTTP 400 is
  returned instead of an HTTP 404.
* Skip tests which create or update an entity, then read it and verify its
  attributes. When fetching a system, an extremely inconsistent set of results
  is returned.
Merge pull request #1228 from Ichimonji10/jenkins

Skip known-failing tests