Fix failing ZuulFilter unit test

As the implementation implies, higher-priority tasks should sort first.
Avoid writing to zuul-core/moo on test runs

The alternative is writing them to some gitignore'd location, but
avoiding hitting the filesystem seems preferable since it's not needed.
Eliminate inexact argument warning

This eliminates the "non-varargs call of varargs method with inexact
argument type for last parameter" warning.
Merge pull request #25 from trptcolin/test_cleanup

Test cleanup