RT 3493: fix RSA test

- Pass in the right ciphertext length to ensure we're indeed testing
  ciphertext corruption (and not truncation).
- Only test one mutation per byte to not make the test too slow.
- Add a separate test for truncated ciphertexts.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(cherry picked from commit 25d6b340)