kunit/memcpy: Add dynamic size and window tests

The "side effects" memmove() test accidentally found[1] a corner case in
the recent refactoring of the i386 assembly memmove(), but missed another
corner case. Instead of hoping to get lucky next time, implement much
more complete tests of memcpy() and memmove() -- especially the moving
window overlap for memmove() -- which catches all the issues encountered
and should catch anything new.

[1] https://lore.kernel.org/lkml/CAKwvOdkaKTa2aiA90VzFrChNQM6O_ro+b7VWs=op70jx-DKaXA@mail.gmail.com

Cc: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>