I have get test failures on `openFile008` with
`openFile: resource exhausted (Too many open files)` when running
inside `./validate`, but not when I run the test individually.
I suspect that's because with `./validate` parallelism of 33 threads
I go just above the `1024` file descriptor limit.
This is probably related to the recent change:
`58f0086b: Testsuite: open/close stdin/stdout/stderr explicitly`
but I haven't looked deep enough to understand exactly how.
I think bumping this is harmless, but I don't really know why it's
necessary at all.

Test Plan: ./validate

Reviewers: austin, thomie, hvr, bgamari, simonmar

Reviewed By: simonmar

Subscribers: simonmar

Differential Revision: https://phabricator.haskell.org/D2368