tests: test-module-imports.t works on windows (with backslash path sep)

Transform the backslashes to slashes on windows, because they are eaten up by
the shell. (Similar as done in test-check-pyflakes.t.)

This test will not produce a traceback on windows anymore. But the test still
fails because the detected mixed imports differ.

tests/test-module-imports.t

@@ -22,7 +22,7 b' here that we should still endeavor to fi'
 hidden by deduplication algorithm in the cycle detector, so fixing
 these may expose other cycles.
 
-  $ hg locate 'mercurial/**.py' | xargs python "$import_checker"
+  $ hg locate 'mercurial/**.py' | sed 's-\\-/-g' | xargs python "$import_checker"
   mercurial/dispatch.py mixed imports
      stdlib:    commands
      relative:  error, extensions, fancyopts, hg, hook, util