Merge pull request from GHSA-3x4c-pq33-4w3q

* Add some tests to characterise the problem

Some failing. Current states:

  RoomsMemberListTestCase
test_get_member_list ...
[OK]
test_get_member_list_mixed_memberships ...
[OK]
test_get_member_list_no_permission ...
[OK]
test_get_member_list_no_permission_former_member ...
[OK]
test_get_member_list_no_permission_former_member_with_at_token ...
[FAIL]
test_get_member_list_no_room ...
[OK]
test_get_member_list_no_permission_with_at_token ...
[FAIL]

* Correct the tests

* Check user is/was member before divulging room membership

* Pull out only the 1 membership event we want.

* Update tests/rest/client/v1/test_rooms.py

Co-authored-by: Erik Johnston <erik@matrix.org>

* Fixup tests (following apply review suggestion)

Co-authored-by: Erik Johnston <erik@matrix.org>