test: add pyflakes checking for unable to detect undefined names

tests/filterpyflakes.py

@@ -21,6 +21,7 b' for line in sys.stdin:'
     pats = [
             r"imported but unused",
             r"local variable '.*' is assigned to but never used",
+            r"unable to detect undefined names",
            ]
     if not re.search('|'.join(pats), line):
         continue

tests/test-check-pyflakes.t

@@ -7,5 +7,9 b''
   mercurial/commands.py:*: 'mpatch' imported but unused (glob)
   mercurial/commands.py:*: 'osutil' imported but unused (glob)
   mercurial/revlog.py:*: 'short' imported but unused (glob)
+  hgext/inotify/linux/__init__.py:*: 'from _inotify import *' used; unable to detect undefined names (glob)
+  mercurial/util.py:*: 'from posix import *' used; unable to detect undefined names (glob)
+  mercurial/windows.py:*: 'from win32 import *' used; unable to detect undefined names (glob)
+  mercurial/util.py:*: 'from windows import *' used; unable to detect undefined names (glob)