generator test uses random source

[simbo1905]

The following test:

 @Test
  void generate() {
    final SecureRandom random = new SecureRandom(); // <- random 
    final byte[] p = GF256.generate(random, 5, (byte) 20);
   // removed for brevity
   assertThat(p[p.length - 1]).isNotZero();
  }

Will find it hard to catch a bug where generate doesn't loop on the random source until p[p.length - 1] != (byte) 0. If a regression is introduced it is unlikely to be caught before the code is released.

[simbo1905]

I have submitted PR 16 for this.