Add test to ensure push/notifications work after room purge #629

Job	Run time
Dendrite: SQLite	1m 35s
Dendrite: Postgres	1m 35s
Synapse: Py 3.8, SQLite 3.31.1, Monolith	1m 32s
Synapse: Py 3.8, PG 12, Monolith	1m 33s
Synapse: Py 3.8, PG 12, Workers	1m 34s
Synapse: Py 3.10, PG 14, Monolith	1m 32s
Synapse: Py 3.10, PG 14, Workers	1m 32s
Synapse: debian-testing, Monolith	1m 34s
Synapse: debian-testing, Workers	1m 33s
	14m 0s

Provide feedback