testsuite: Fix putStrLn in saks028

Bizarrely, `saks028` previously failed reliably, but only on Windows
(#17450). The test would exit with a zero exit code but simply didn't
emit the expected text to stderr.

I believe this was due to the fact that the test used `putStrLn`,
resulting in the output ending up on stdout. This worked on other
platforms since (apparently) we redirect stdout to stderr when
evaluating splices. However, on Windows it seems that the redirected
output wasn't flushed as it was on other platforms.

Anyways, it seems like the right thing to do here is to be explicit
about our desire for the output to end up on stderr.

Closes #17450.