sytest `Local device key changes appear in /keys/changes` flakes

[DMRobertson]

In matrix-org/sytest#1129 (comment) we noticed that a specific sytest that would intermittently fail, but seemingly only for dendrite running without full HTTP APIs.

In that PR I wasn't modifying dendrite---just sytest's GHA config. It should have been fetching dendrite's master branch throughout. Not sure if there's a flakey test or a problem in dendrite? @anoadragon453 suggested I get it down in writing here. Happy to move it to sytest if we can confirm it's not a dendrite problem.

Some specific sytest runs:

• https://github.com/matrix-org/sytest/runs/3525244956
• https://github.com/matrix-org/sytest/runs/3524830553
• https://github.com/matrix-org/sytest/runs/3527233610
• https://github.com/matrix-org/sytest/actions/runs/1215486987 (failing on sqlite)
• https://github.com/matrix-org/sytest/pull/1134/checks?check_run_id=3566373941 failing on sqlite no full http; postgres + full http

And also seen on buildkite:

• https://buildkite.com/matrix-dot-org/sytest/builds/1581#b1e10cd8-9a02-4e9f-911b-1d53571effca

Background information

• Dendrite version or git SHA: "Dendrite version 0.5.0"
• Monolith or Polylith?: monolith
• SQLite3 or Postgres?: Postgres
• Running in Docker?: yes
• go version: unsure

[S7evinK]

Going to close this, haven't seen this one fail in quite some time.
Also removed the test from our blacklist, as we might not have seen any failures because of that. (I've checked several recent runs and didn't see it fail, though)