test: Generate random, increasing message IDs for example data by default

[gnprice]

This allows eg.streamMessage and eg.dmMessage to be used in
contexts where it's important that the message IDs don't collide,
or even that they're increasing, without the test having to explicitly
give specific message IDs when no other fact about them is relevant.

As a result, we get to cut numerous explicit message IDs that are now redundant.

[gnprice]

I ran flutter test 100 times on this with no failures:

$ time { for i in {1..100}; do flutter test; done; }
00:09 +655 ~7: All tests passed!                                                 
[… 98 more successes …]
00:10 +655 ~7: All tests passed!                                                 

time: 1128.545s wall (3879.761s u, 898.730s s)

So it looks like we don't have any existing tests that flake from this randomness. I don't think there are any super likely patterns we'd use that would introduce such flakes, but if we start seeing them then we can adjust how this works.

[gnprice]

(This was prompted by #317 (comment) — an existing test that wasn't passing explicit message IDs started to need them, because an unrelated change caused more of the app to start caring that the IDs of different messages were different. After this PR, that change doesn't affect that test.)

[chrisbobbe]

Thanks, this is helpful! Merged.