I'm not sure if the GetTempPathW() dir is okay for this. d3dx11 seems to do the same in tests, so I copied that approach.
Commit message:
The wine testbot runs tests with C:\Users\Public\Documents as the working directory. Itemdlg tests create test files in the working directory and then try to select them for opening in an IFileOpenDialog. The dialog has its SetFolder method called to point it to the current working directory before showing the dialog, in order for the test code to find the test files it had created there just before opening the dialog.
On Windows, this failed because calling the dialog's SetFolder method with C:\Users\Public\Documents doesn't work as expected. The dialog points to the C:\Users\[CURRENT_USER]\Documents directory instead. Subsequently, the test would be unable to select the test file because it doesn't exist in that directory. So the dialog wouldn't accept the file and close itself to finish the test, but show an error message instead, resulting in a test timeout.
To fix this, set the current working directory to the temporary directory before calling individual itemdlg test_* functions. Switch back to the previous working directory after tests finish.