warnings and pretty-printer output sometimes seem to mix on FreeBSD

[catamorphism]

This causes tests to fail pretty-printing on the FreeBSD buildbot, seemingly at random. Possibly a problem with how to redirect stderr and stdout.

[catamorphism]

Should be fixed by #2523, which I've merged, but waiting for the tests to finish running.

[catamorphism]

FreeBSD tests pass, so I'm considering this done, thanks to @crabtw .