harness: replace decorate with Logger
harness: replace decorate with Logger

Later on the `output` buffer can become a plain *os.File in order to
record per-test logs. Using a full logger removes code duplication and
opens the possibility of passing it outside of the test function.

marineam/mantle,glevand/coreos--mantle,quantum/coreos-mantle,quantum/coreos-mantle,marineam/mantle,dm0-/mantle,dm0-/mantle,glevand/coreos--mantle