We need a test case that shows that the scanner occasionally tries the primary provider.

Maybe something like:

1. Spawn anvil_1
2. Connect provider_1
3. Store anvil_1.port() in a local variable anvil_1_port
4. Spawn anvil_2
5. Connect provider_2
6. Mint block on anvil_1, assert
7. Drop anvil_1
8. Mint block on anvil_2, assert stream still works
9. Spawn new anvil_3 with .port(anvil_1_port) <- this will ensure provider_1 ws_endpoint_url is again valid
10. ???
11. Verify that scanner switches back to provider_1

1. Add test with multiple pubsub fallbacks, where each works but then stops, to confirm scanner goes through all of them in turn.
2. Add subscribe test with no fallbacks