Add test to ensure push/notifications work after room purge #632

Job	Run time
Dendrite: SQLite	9m 9s
Dendrite: Postgres	9m 51s
Synapse: Py 3.8, SQLite 3.31.1, Monolith	8m 1s
Synapse: Py 3.8, PG 12, Monolith	13m 10s
Synapse: Py 3.8, PG 12, Workers	17m 21s
Synapse: Py 3.10, PG 14, Monolith	16m 47s
Synapse: Py 3.10, PG 14, Workers	27m 10s
Synapse: debian-testing, Monolith	10m 50s
Synapse: debian-testing, Workers	23m 24s
	2h 15m 43s

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