And canonicalise file paths, so that output is consistent, even in
out-of-tree builds.
Capturing stderr output produces better diagnostics.
Diagnostics for 32705.
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.
