libio: Update license on test case

About a decade ago, I accidentally wrote the GPLv3 license text on the
test case when the rest of glibc source is LGPL v2.1 or later.  As
original author of the test (and there are no other legally
significant changes to the test) I propose to update the license text
to be consistent with the project.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>