sliding sync: Add test to verify we send unsubscribe_rooms for every room switch

[S7evinK]

This adds a test to verify that we send unsubscribe_rooms for every room switch.
CI is going to fail for this test, as of now, as it needs matrix-org/matrix-js-sdk#2991

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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This change is marked as an internal change (Task), so will not be included in the changelog.

[kegsay]

I didn't know you could do this in Cypress! Might need a check from @t3chguy with his Cypress-fu, but this LGTM.

[t3chguy]

Looks sane to me, but in future if you name your branches the same then the CI will run them against each other, presumably the fail right now is due to the js-sdk changeset not being considered. Also might be worth probably checking the content of the room_subscriptions - to ensure the wrong room isn't being unsubbed?

[richvdh]

looks like the new test is failing

[richvdh]

oh I see, that's expected. Still, we can't merge this until it is unblocked by matrix-org/matrix-js-sdk#2991.

[S7evinK]

oh I see, that's expected. Still, we can't merge this until it is unblocked by matrix-org/matrix-js-sdk#2991.

Thanks for approving this one, now lets wait for CI. :)