Refactor REST API tests to use explicit reactors (#3351)

changelog.d/3351.misc

synapse/http/site.py

synapse/rest/client/v2_alpha/register.py

tests/rest/client/v1/test_register.py

tests/rest/client/v1/test_rooms.py

tests/rest/client/v1/utils.py

tests/rest/client/v2_alpha/__init__.py

tests/rest/client/v2_alpha/test_filter.py

tests/rest/client/v2_alpha/test_register.py

tests/rest/client/v2_alpha/test_sync.py

tests/server.py

tests/test_server.py

tests/unittest.py