Autopackage tests will not signal failure when failing

As discovered by Michael Terry the DEP8 tests are not going to fail when actual problem is encountered:
"""
2) The dep8 test will never fail, because the test run is inside an if condition. You should change the following lines:

if plainbox self-test -u >/dev/null 2>&1; then
    echo "unit-tests: OK"
fi

to just (which works as intended because of the set -e):

plainbox self-test -u >/dev/null 2>&1
echo "unit-tests: OK"
"""