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_balance_shift_sites.py

@@ -1,6 +1,6 @@
 import pytest
 import datetime
-from rota.shifts import RotaBuilder, SingleShift, days
+from rota.shifts import BalanceAcrossGroupsConstraint, RotaBuilder, SingleShift, days
 from rota.workers import Worker
 
 @pytest.fixture
@@ -31,7 +31,7 @@ def test_balance_blocks_across_2_groups(demo_rota_balance_sites):
             balance_offset=40,
             workers_required=2,
             force_as_block=True,
-            constraint=[{"name": "balance_across_groups"}],
+            constraints=[BalanceAcrossGroupsConstraint()],
         ),
     )
     Rota.constraint_options["balance_nights_across_sites"] = False
@@ -76,7 +76,7 @@ def test_balance_blocks_across_2_groups_unbalanced(demo_rota_balance_sites):
             balance_offset=40,
             workers_required=3,
             force_as_block=True,
-            constraint=[{"name": "balance_across_groups"}],
+            constraints=[BalanceAcrossGroupsConstraint()],
         ),
     )
     Rota.constraint_options["balance_nights_across_sites"] = False
@@ -116,7 +116,7 @@ def test_balance_blocks_across_3_groups(demo_rota_balance_sites):
             balance_offset=40,
             workers_required=3,
             force_as_block=True,
-            constraint=[{"name": "balance_across_groups"}],
+            constraints=[BalanceAcrossGroupsConstraint()],
         ),
     )
     Rota.constraint_options["balance_nights_across_sites"] = False