String: Strength memset tests in test-memset.c

The prior sentinel logic was broken and was checking the SIMPLE_MEMSET
as opposed to the tested implementation. As well `s` (the test buffer)
was not reset between implementation tests so it was possible for a
buggy implementation to be hidden by a previously executed correct
one.
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>