test-pyflakes: detect undefined name error
It should be able to catch the following mistakes at
2606e7f227f6:
mercurial/exchange.py:590: undefined name 'UnknownPartError'
mercurial/match.py:346: undefined name 'pat'
mercurial/win32.py:365: undefined name '_ERROR_NO_MORE_FILES'
tests/killdaemons.py:46: undefined name 'check'