shlwapi: add some more tests for UrlUnEscape

[Louis. Lenders]

hi, tests show that UrlUnEscape still converts the url-string if the bufferpointer is set to NULL, and the 
URL_UNESCAPE_INPLACE is set. Test passes fine with native shlwapi, and in wine after applying the previous sent patch :

http://www.winehq.org/pipermail/wine-patches/attachments/20080814/d5feb31b/attachment.txt

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