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_general_balancing.py

@@ -1,7 +1,7 @@
 import datetime
 import pytest
 from pytest import approx
-from rota.shifts import RotaBuilder, SingleShift, days
+from rota.shifts import PostShiftConstraint, PreShiftConstraint, RotaBuilder, SingleShift, days
 from rota.workers import Worker
 
 def generate_basic_rota(weeks_to_rota=10):
@@ -61,7 +61,7 @@ def test_weighted_shift_balancing():
             balance_weighting=10,
             workers_required=1,
             force_as_block=False,
-            constraint=[{"name": "pre","options": "2"}, {"name": "post","options": "2"}],
+            constraints=[PreShiftConstraint(days=2), PostShiftConstraint(days=2)]
         ),
         SingleShift(
             sites=("group1", "group2"),