Test that the encrypted payloads are uniform.

[DanGould]

close #371

This randomized test will generate a false negative with negligible probability
if all encrypted messages share an identical byte at a given position by chance.
It should fail deterministically if any bit position has a fixed value.

re #364 (review) from @nothingmuch

I did check that this test would indeed fail before the ellswift changes by cherry-picking the test on 9c4880c

[nothingmuch]

ACK with some minor corrections, IMO can be merged with or without my last proposed change (bit-wise instead of byte-wise comparison) at your discretion (if you do then make sure to adjust the number of messages I didn't add a suggestion for that).

[nothingmuch]

Another approach would be to check that all bits (as opposed to bytes) contain variation. For this check the number of messages in the set should be 80 or 128 or whatever instead of 16 in order to have negligible chance of false positive.

Suggested change

	accumulator.iter().all(|&b| b != 0),
	accumulator.iter().any(|&b| b != 0xff),

edit: previous suggestion was incorrect, comparing to 1 instead of 0xff

[DanGould]

slight typo in your suggestion:

!accumulator.iter().any(|&b| b != 0xff) == accumulator.iter().all(|&b| b == 0xFF).

I chose to implement the latter for readability.

[nothingmuch]

oof,3rd time i'm trying to post this "pending" comment, not sure what went wrong the first two times.... sorry for the delay