WIP collectfile: improve assertion [ci skip]

[blueyed]

Ref: #4788 (comment)

[nicoddemus]

Thanks!

[blueyed]

Meant for testing.. let's wait for feedback from #4788 (comment).

[rouault]

OK, more interesting info from the assert branch:

c:\python27\lib\site-packages\_pytest\runner.py:226: in from_call
    result = func()
c:\python27\lib\site-packages\_pytest\runner.py:289: in <lambda>
    call = CallInfo.from_call(lambda: list(collector.collect()), "collect")
c:\python27\lib\site-packages\_pytest\python.py:655: in collect
    for x in self._collectfile(path):
c:\python27\lib\site-packages\_pytest\python.py:606: in _collectfile
    path.islink(),
E   AssertionError: local('C:\\projects\\gdal\\autotest\\pyscripts\\data\\??') is not a file (isdir=False, exists=False, islink=False)

It turns out that this '??' file is the autotest/pyscripts/data/漢字/ directory that couldn't be transcripted to the Windows filesystem encoding while checking out from git

[blueyed]

@rouault
Thanks!

So I think the fix and the more verbose assertion is fine. Integrated in #4784.