Investigate sparkline tests

[vks]

The sparkline tests are much more likely to fail than the statistical assumptions of the test suggest. This should be better understood by studying the error distribution.

I modified the test by feeding all the numbers in the range 0..100 to the seed_from_u64 and these seeds failed at least one expected error bucket.

1 2 3 8 10 12 14 15 25 38 44 46 48 49 54 63 66 73 81 82 87 93

This means that 22/100 random seeds failed the unit test with error more than 3 standard deviations from expected.
If I allow 4 standard deviations, only a single seed fails 87
I get similar results if I use from_entropy or switch back to the pcg32 implementation.

The other solution would ditch the truly obvious seed of 1 and pick a fair random number like 4 to move on.

Originally posted by @ironhaven in #1203 (comment)

[dhardy]

Worth noting that this is using a million samples. It would be useful to calculate the chance of accidental failure.