arm: remove string/tst-memmove-overflow XFAIL

The arm string/tst-memmove-overflow XFAIL has been added in commit
eca1b233 ("arm: XFAIL string/tst-memmove-overflow due to bug 25620")
as a way to reproduce the reported bug.

Now that this bug has been fixed in commits 79a4fa34 ("arm:
CVE-2020-6096: fix memcpy and memmove for negative length [BZ #25620]")
and beea3610 ("arm: CVE-2020-6096: Fix multiarch memcpy for negative
length [BZ #25620]"), let's remove the XFAIL.

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