test_overflow: Add shift overflow tests

This adds overflow tests for the new check_shift_overflow() helper to
validate overflow, signedness glitches, storage glitches, etc.

Co-developed-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>