insane.bbclass: add host-user-contaminated test

insane.bbclass: add host-user-contaminated test

- Add a test which checks for any paths outside of /home which are owned by
  the user running bitbake.
- Add the test to WARN_QA by default.

This test has been in meta-mentor for some time, and in our ERROR_QA for our
builds, and has caught a number of issues for us.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>