test-debugextensions: sanitize JSON output for Windows

Windows is printing a double backslash, which isn't handled by (glob):

  --- c:/Users/Matt/Projects/hg/tests/test-debugextensions.t
  +++ c:/Users/Matt/Projects/hg/tests/test-debugextensions.t.err
  @@ -47,7 +47,7 @@
      {
       "buglink": "",
       "name": "color",
  -    "source": "*/hgext/color.pyc", (glob)
  +    "source": "c:\\Users\\Matt\\Projects\\hg\\hgext\\color.pyc",
       "testedwith": "internal"
      },
      {

tests/test-debugextensions.t

@@ -42,7 +42,7 b''
     location: */hgext/rebase.pyc (glob)
     tested with: internal
 
-  $ hg debugextensions -Tjson
+  $ hg debugextensions -Tjson | sed 's|\\\\|/|g'
   [
    {
     "buglink": "",