Instead, return the values of the test and benchmark setup data files so
that the ninja/osx/vs backends can use those filenames instead of
hard-coding them.
This makes the following changes:
* Explicitly closes the file for force flushing (this fixes an issue on which the last 2 or 3 lines weren't being written to disk)
* Adds another check on the PBXBuildFile stage to get the file name if the returned source type is a File instead of a string
Fixes issue #337
Nirbheek Chauhan