[3/4] (Try4) msi/tests: Fix test when run on a different drive than C:\.

[Nicolas Le Cam]

Testing a little bit more the three other patches I found that running
tests on an other drive than the SystemDrive one was returning strange
results.

When run on a different drive than SystemDrive in pathes that are also
present in SystemDrive (tested in root dir and in Documents And Settings
\user\Desktop one), MsiGetPropertyA returns pathes on SystemDrive. While
it returns empty string if path doesn't exist on SystemDrive (tested in
a temp dir).

Tested on Win2k/WinXP, marked todo_wine as it doesn't pass actually on
wine.

Nicolas Le Cam