Fix addFailTest

The addFailTest method needed to be updated to catch up with some
test infrastructure changes.  Rather than consulting the merger's
git repo for the ZUUL_REF to find out if a job which is supposed to
fail is present for a given build, use the new FakeBuild.hasChanges
method to inspect the launcher's prepared git repo.

Also rename the method as 'Job' is more descriptive than 'Test' and
document it.

Change-Id: I3224b8a01d49cfa06b799a8028c1bf0d455d25b1
3 files changed