private byte[] reverse(byte[] x) {
    byte[] r = new byte[x.length];
    for (int i = 0; i < x.length; i++) {
      r[i] = x[x.length - i - 1];
    }
    return r;
  }
The idea of basic tests is simple. We generate random numbers, make computations with
Field25519 and compare the results with Java BigInteger.
java_src/src/test/java/com/google/crypto/tink/internal/Field25519Test.java