Ensure that start block for DKG event scanning is never negative.

Stop RitualTracker when Ursula stops.
Cleanup test.

nucypher/blockchain/eth/trackers/dkg.py

@@ -134,7 +134,7 @@ class ActiveRitualTracker:
         )
 
         expected_start_block = w3.eth.get_block(
-            latest_block.number - number_of_blocks_in_the_past
+            max(0, latest_block.number - number_of_blocks_in_the_past)
         )
         while (
             expected_start_block.number > 0

nucypher/characters/lawful.py

@@ -1023,6 +1023,7 @@ class Ursula(Teacher, Character, Operator, Ritualist):
             self.stop_learning_loop()
             self.work_tracker.stop()
             self._operator_bonded_tracker.stop()
+            self.ritual_tracker.stop()
         if halt_reactor:
             reactor.stop()
 

tests/acceptance/characters/test_operator.py

@@ -12,18 +12,9 @@ from tests.utils.middleware import NodeIsDownMiddleware
 from tests.utils.ursula import make_ursulas
 
 
-@pytest.mark.usefixtures("ursulas")
-def test_stakers_bond_to_ursulas(
-    testerchain, test_registry, staking_providers, ursula_test_config
-):
-    nodes = make_ursulas(
-        ursula_config=ursula_test_config,
-        staking_provider_addresses=testerchain.stake_providers_accounts,
-        operator_addresses=testerchain.ursulas_accounts,
-    )
-
-    assert len(nodes) == len(staking_providers)
-    for ursula in nodes:
+def test_stakers_bond_to_ursulas(ursulas, test_registry, staking_providers):
+    assert len(ursulas) == len(staking_providers)
+    for ursula in ursulas:
         ursula.validate_operator(registry=test_registry)
         assert ursula.verified_operator