tests: force `dumbhttp.py` to write its log file with '\n' on Windows
This wasn't causing obvious test failures, but it's the same fix as
dbd2d56224d1
for `dummysmtpd.py`, and there's no sense in leaving this problem lying around.
(And upon further review, it might have been causing some non-obviously related
failures- see the next commit.)
(binary:application/octet-stream)