tests: Write temporary files to executable folder.

Temp folder is the root folder on Windows; writing to the folder may require administrator rights.
We can't use current folder for temporaries because tests from different configurations can be running
in parallel, but executable folder is always safe since we only run each executable once.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@984 99668b35-9821-0410-8761-19e4c4f06640