Tests: adjusted todo_alerts() in mirror_proxy.t on win32.

The test producing alert was previously skipped on this platform.
diff --git a/mirror_proxy.t b/mirror_proxy.t
index 5f94f03..3c52c42 100644
--- a/mirror_proxy.t
+++ b/mirror_proxy.t
@@ -102,7 +102,7 @@
 
 }
 
-$t->todo_alerts();
+$t->todo_alerts() unless $^O eq 'MSWin32';
 $t->stop();
 
 my $log = $t->read_file('test.log');