The testsuite driver has a little known feature to check which files
each test writes to, whether there are tests that write to same file,
and whether the tests leave any files behind when CLEANUP=1. It uses
strace under the hood.

This commit fixes some bitrot, and filters out some more strace lines
that we're not interested in (and are shown as framework failures
otherwise).

Differential Revision: https://phabricator.haskell.org/D979