Test unit length compliance (255 bytes vs unicode points)

[ShadowJonathan]

This follows https://github.com/matrix-org/matrix-spec/issues/1001, I think it'd be good to add a test that essentially instigates this behaviour, trying out strings of emojis that are less than 255 codepoints, but are more than 255 bytes.

The only question left is how many of the different fields we're going to test, I'd suggest to at least check:

• type
• state_key
• user_id

[neilalexander]

And sender.