TST: fix portability issue with new serialization tests - #19634

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I was looking at this earlier (but didn't get to coding yet) and my thought was just use ellipsis in the doc output. But if this works, I guess it is better. Thanks!

But if this works, I guess it is better

It doesn't, actually. I think this means it's a real bug, so we shouldn't hide it away with ellision either.

Can you really force a int64 on a 32-bit emulator?

Yes. It's just not used by default. I suspect this means we don't actually serialize data types correctly on any platform, but tests happen to work on all 64-bit archs

Actually, my patch (or a variation of it, including deliberate use of non-native precisions) would probably be needed to test the correct solution any way, so I'll re-open as a draft.

closing in favor or #19644