random: Add entropy test.

random: Add entropy test.

This test would have made the bug fixed in the previous commit obvious.  It
would have printed the following:

average=0007c220

bit      0     1
  0  5012  4988
  1  5019  4981
  2  5154  4846
  3  4909  5091
  4  5011  4989
  5  5021  4979
  6  4911  5089
  7  4910  5090
  8  5011  4989
  9  5020  4980
 10  5154  4846
 11  5021  4979
 12  5155  4845
 13  5019  4981
 14  5153  4847
 15  5153  4847
 16  5153  4847
 17  5153  4847
 18  5153  4847
 19  5152  4848
 20 10000     0
 21 10000     0
 22 10000     0
 23 10000     0
 24 10000     0
 25 10000     0
 26 10000     0
 27 10000     0
 28 10000     0
 29 10000     0
 30 10000     0
 31 10000     0
(expected values are 5000)

nibble   0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
     0 696 646 564 693 542 545 612 611 687 663 645 560 674 566 592 704
     1 625 589 597 571 638 623 604 663 670 652 650 557 541 683 686 651
     2 628 644 659 672 648 569 585 616 623 647 681 600 586 675 601 566
     3 683 665 717 549 633 613 701 592 663 531 545 800 623 608 590 487
     4 622 657 777 557 720 608 613 598 657 678 551 654 615 596 598 499
     5 10000   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0
     6 10000   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0
     7 10000   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0
(expected values are 625)