tests: fix `hghave root` on windows

This lets tests\test-hghave.t pass on windows where geteuid does not exist.

tests/hghave.py

@@ -234,7 +234,7 b' def has_unix_permissions():'
         os.rmdir(d)
 
 def has_root():
-    return os.geteuid() == 0
+    return getattr(os, 'geteuid', None) and os.geteuid() == 0
 
 def has_pyflakes():
     return matchoutput("sh -c \"echo 'import re' 2>&1 | pyflakes\"",