Fix switch_stdout_to on Windows7

The switch_stdout_to test was broken on Windows7, as the test
infrastructure would refuse to delete the temporary test folder because
the switch-stdout-output file we redirected the stdout to was still
opened.

To fix this issue, we make switch_stdout_to return the previous handle,
and add a new switch_stdout_to call at the end of the test to return the
stdio handles to their original state. The handle the second
switch_stdout_to returns will be automatically closed, which should
allow the temporary test folder to be deleted properly.