Check eager sync behaviour after leave then un-partial-stating

[matrixbot]

This issue has been migrated from #15090.

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• Per Non lazy loading sync not blocking during fast join matrix-org/synapse#14831 (comment), we should write to a complement test which checks that eager syncs don't see anything when the room becomes un-partial-stated after we leave.

Originally posted by @DMRobertson in matrix-org/synapse#12802 (comment)