test-check-config: convert directory separators to '/' for MSYS

The output of the files command uses native separator.  MSYS then seems to drop
the '\' on Windows when invoking python:

 --- c:/Users/Matt/Projects/hg/tests/test-check-config-hg.t
 +++ c:/Users/Matt/Projects/hg/tests/test-check-config-hg.t.err
 @@ -6,22 +6,10 @@

    $ hg files "set:(**.py or **.txt) - tests/**" |
    >   xargs python contrib/check-config.py  Traceback (most recent call last):
      File "contrib/check-config.py", line 93, in <module>
        sys.exit(main(sys.argv[1:]))
      File "contrib/check-config.py", line 24, in main
        for l in open(f):
    IOError: [Errno 2] No such file or directory: 'contriball-revsets.txt'
    [123]

tests/test-check-config-hg.t

@@ -4,7 +4,7 b''
 
 New errors are not allowed. Warnings are strongly discouraged.
 
-  $ hg files "set:(**.py or **.txt) - tests/**" |
+  $ hg files "set:(**.py or **.txt) - tests/**" | sed 's|\\|/|g' |
   >   xargs python contrib/check-config.py
   undocumented: convert.cvsps.cache (bool) [True]
   undocumented: convert.cvsps.fuzz (str) [60]