lib/crypto: tests: Add Poly1305 split-update carry regression test

Add a test case which reproduces the bug fixed by commit 24f07e8c07a0
("lib/crypto: arm64: Fix lost Poly1305 carry when resuming NEON state").
It is written in a fairly general way, such that it should also be able
to reproduce similar bugs in other implementations.

Update the comment on test_poly1305_allones_keys_and_message() to more
accurately describe what it does and differentiate it from the new test.

Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://patch.msgid.link/20261009125354.315476-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>