testing/quick: generate all possible int64, uint64 values
When generating a random int8, uint8, int16, uint16, int32, uint32,
quick.Value chooses among all possible values.
But when generating a random int64 or uint64, it only chooses
values in the range [-2⁶², 2⁶²) (even for uint64).
It should, like for all the other integers, use the full range.
If it had, this would have caught #19807 earlier.
Instead it let us discover the presence of #19809.
While we are here, also make the default source of
randomness not completely deterministic.