Refactor shift constraints in tests to use new constraint classes

- Updated test cases in `test_workers.py` to replace dictionary-based constraints with instances of `PreShiftConstraint`, `PostShiftConstraint`, and `MaxShiftsPerWeekConstraint`.
- Added a new test `test_worker_assign_as_block_preference2` to verify worker preferences for block assignments.
- Marked tests as slow where appropriate to improve test suite performance.

test/test_night_unavailable.py

@@ -1,6 +1,6 @@
 import pytest
 import datetime
-from rota.shifts import RotaBuilder, SingleShift, days
+from rota.shifts import NightConstraint, RotaBuilder, SingleShift, days
 from rota.workers import Worker
 
 @pytest.fixture
@@ -45,7 +45,7 @@ def test_basic_assignment(demo_rota_night_unavailable):
             name="night_weekend",
             length=12.5,
             days=days[5:],
-            constraint=[{"name": "night"}],
+            constraints=[NightConstraint()],
         ),
     )
     Rota.build_and_solve(options={"ratio": 0.00})
@@ -66,7 +66,7 @@ def test_assign_night_prior_to_unavailablity(demo_rota_night_unavailable):
             length=12.5,
             days=days[5:],
             workers_required=2,
-            constraint=[{"name": "night"}],
+            constraints=[NightConstraint()],
         ),
     )
     Rota.build_and_solve(options={"ratio": 0.00})
@@ -101,7 +101,7 @@ def test_assign_prior_to_unavailablity_non_night(demo_rota_night_unavailable):
             length=12.5,
             days=days[5:],
             workers_required=2,
-            constraint=[],
+            constraints=[],
         ),
     )
     Rota.build_and_solve(options={"ratio": 0.00})
@@ -135,7 +135,7 @@ def test_assign_split(demo_rota_night_unavailable):
             length=12.5,
             days=days[5:],
             workers_required=3,
-            constraint=[],
+            constraints=[],
         ),
     )
     Rota.build_and_solve(options={"ratio": 0.00})