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.)

tests/dumbhttp.py

@@ -5,6 +5,7
 Small and dumb HTTP server for use in tests.
 """
 
+import io
 import optparse
 import os
 import signal
@@ -21,6 +22,20 from mercurial import (
 httpserver = util.httpserver
 OptionParser = optparse.OptionParser
 
+if pycompat.iswindows:
+    sys.stdout = io.TextIOWrapper(
+        sys.stdout.buffer,
+        sys.stdout.encoding,
+        sys.stdout.errors,
+        newline="\n",
+    )
+    sys.stderr = io.TextIOWrapper(
+        sys.stderr.buffer,
+        sys.stderr.encoding,
+        sys.stderr.errors,
+        newline="\n",
+    )
+
 if os.environ.get('HGIPV6', '0') == '1':
 
     class simplehttpserver(httpserver.httpserver):