Raise testrepository requirement to >= 0.0.15

It has full python3 support and actually fixes a weird
exception in utf8 encoding/decoding. Only noteworthy
change is documented in NEWS:

``--full-results`` is now a no-op, use ``--subunit`` to get unfiltered
  output. (Robert Collins)

Change-Id: I8c1393371e80338179135c416d93a68d7879af4f
Merge "Raise testrepository requirement to >= 0.0.15"