message: [crypto] making x25519 tests use TEST_SEED
  Closes:
  Approved by: philp9
diff: //! # Examples
  //!
  //! ```
  -//! use libra_crypto::{x25519, Uniform};
  +//! use libra_crypto::{x25519, Uniform, test_utils::TEST_SEED};
  //! use rand::{rngs::StdRng, SeedableRng};
  //!
  //! // Derive an X25519 private key for testing.
  -//! let seed = [1u8; 32];
  -//! let mut rng: StdRng = SeedableRng::from_seed(seed);
  +//! let mut rng: StdRng = SeedableRng::from_seed(TEST_SEED);
  //! let private_key = x25519::PrivateKey::generate(&mut rng);
  //! let public_key = private_key.public_key();
  //!
  