lib/crypto: tests: Migrate ChaCha20Poly1305 self-test to KUnit

lib/crypto: tests: Migrate ChaCha20Poly1305 self-test to KUnit

Move the ChaCha20Poly1305 test from an ad-hoc self-test to a KUnit test.

Keep the same test logic for now, just translated to KUnit.

Moving to KUnit has multiple benefits, such as:

- Consistency with the rest of the lib/crypto/ tests.

- Kernel developers familiar with KUnit, which is used kernel-wide, can
  quickly understand the test and how to enable and run it.

- The test will be automatically run by anyone using
  lib/crypto/.kunitconfig or KUnit's all_tests.config.

- Results are reported using the standard KUnit mechanism.

- It eliminates one of the few remaining back-references to crypto/ from
  lib/crypto/, specifically a reference to CONFIG_CRYPTO_SELFTESTS.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20260327224229.137532-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>