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_rota_nights.py

@@ -1,8 +1,9 @@
 import datetime
 import pytest
 from pytest import approx
-from rota.shifts import RotaBuilder, SingleShift, days
+from rota.shifts import PostShiftConstraint, RotaBuilder, SingleShift, days
 from rota.workers import Worker
+from web.rota.shifts import PreShiftConstraint
 
 @pytest.fixture
 def demo_rota_nights():
@@ -48,7 +49,10 @@ def demo_rota_nights():
             length=12.5,
             days=days[:4],
             workers_required=1,
-            constraint=[{"name": "pre", "options": 1}, {"name": "post", "options": 2}],
+            constraints=[
+                PreShiftConstraint(days=1),
+                PostShiftConstraint(days=2),
+            ],
             force_as_block=True
         ),
         SingleShift(
@@ -56,7 +60,10 @@ def demo_rota_nights():
             name="night_weekend",
             length=12.5,
             days=days[4:],
-            constraint=[{"name": "pre", "options": 1}, {"name": "post", "options": 2}],
+            constraints=[
+                PreShiftConstraint(days=1),
+                PostShiftConstraint(days=2),
+            ],
             force_as_block=True,
         ),
         SingleShift(