Thread a reactor through PaymentController so tests don't use the real one

src/_zkapauthorizer/_plugin.py

src/_zkapauthorizer/controller.py

src/_zkapauthorizer/resource.py

src/_zkapauthorizer/tests/fixtures.py

src/_zkapauthorizer/tests/test_client_resource.py

src/_zkapauthorizer/tests/test_controller.py

src/_zkapauthorizer/tests/test_plugin.py