Add some basic regression tests to verify behavior of both uint128
implementations at typical boundary conditions, to run on all architectures.
Test uint128 increment behavior of Chacha in keystream mode, as used by
'kern.random.use_chacha20_cipher=1' to verify assumptions. These
assumptions are critical to the safety of using Chacha as a PRF in Fortuna
(as implemented), but do not impact arc4random at all.