Add test for state_key size

[S7evinK]

Part of https://github.com/matrix-org/complement/issues/531

Dendrite would fail to parse events with e.g. state keys with > 255 bytes. This could break rooms and end in endless loops.

Dendrite PR

Failing test here: https://github.com/matrix-org/complement/actions/runs/5484535830/jobs/9992205068?pr=642#step:8:835 Passing test in Dendrite: https://github.com/matrix-org/dendrite/actions/runs/5484587106/jobs/9992396957#step:7:616

[DMRobertson]

Xref https://github.com/matrix-org/matrix-spec/issues/1001