When practracker is disabled, its output will be empty. We don't
want that happening during our tests.
Fixes bug 32705; bugfix on 0.4.2.1-alpha, when test_practracker.sh
was introduced.
This test runs practracker with a set of 0 thresholds, to make sure
that it enumerates all its values right. It tries running with an
empty exceptions file, and with an exceptions file that covers
_some_ of the data, and it makes sure that the outputs are as expected.