The join-trigger documentation still uses the deprecated
"even-if-unstable" field in its examples.
Update the "join-trigger001.yaml" fixture referenced by the
documentation to use the "threshold" configuration instead.
Add a new fixture to retain coverage for "even-if-unstable".
Change-Id: Ic8b28b384e0a6d7e43833d2db9524a41e4508ba7
ac9861d2fb