run-tests: initialize global variables to suppress pyflakes warning
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'