Instead of saying "a non-open change", use the actual status of the
change, i.e.:
cannot set abandoned change to private
cannot set merged change to private
Adjust the setMergedChangePrivate test accordingly, and also add a
new test to cover the abandoned change use case.
Change-Id: I896f20b68e82edf1341c898ab2c0ba680cde1d69
61b8feff59