py3: add a missing b'' for Windows

I tried ./contrib/byteify-strings.py, but there were way too many changes (and
most looked wrong).  This was hit with test-check-interfaces.py.

# skip-blame for b'' prefixes

tests/run-tests.py

@@ -1153,7 +1153,7 b' class Test(unittest.TestCase):'
             killdaemons(env['DAEMON_PIDS'])
             return ret
 
-        output = ''
+        output = b''
         proc.tochild.close()
 
         try:
@@ -1177,7 +1177,7 b' class Test(unittest.TestCase):'
             output = re.sub(s, r, output)
 
         if normalizenewlines:
-            output = output.replace('\r\n', '\n')
+            output = output.replace(b'\r\n', b'\n')
 
         return ret, output.splitlines(True)