Tests for MSC3871: Gappy timelines

[MadLittleMods]

Tests for MSC3871: Gappy timelines

Synapse PR: https://github.com/element-hq/synapse/pull/18873

Dev notes

Document how to hook up Element to the resultant homeservers from Complement: https://github.com/matrix-org/complement/pull/164

matrix-org/complement -> ONBOARDING.md -> How do I hook up a Matrix client like Element to the homeservers spun up by Complement after a test runs?

Todo

• [ ] Create engineered tests using fake Complement homeserver

Pull Request Checklist

• [ ] Pull request includes a sign off