Fix giomm_simple test on Windows

Hi,

From the commit message:

One normally cannot attempt to remove (delete) a file on Windows if it is still open, so we must close the resources that are tied to the file before attempting to delete it.

Without doing so, the test program will fail on Windows as an exception is caught as the file->remove() call failed since the associated iostream is still open.

This will fix the giomm_simple_test test on Windows.

With blessings, thank you!