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_remote.py

@@ -1,6 +1,6 @@
 
 import pytest
-from rota.shifts import NoWorkers, RotaBuilder, SingleShift, days
+from rota.shifts import NoWorkers, RequireRemoteSitePresenceConstraint, RotaBuilder, SingleShift, days
 
 import datetime
 from rota.workers import Worker
@@ -34,7 +34,7 @@ class TestRemoteRotas:
                 workers_required=1,
                 balance_offset=100,
                 force_as_block=False,
-                constraint=[{"name":"require_remote_site_presence_week", "options":("group2", 2)}],
+                constraints=[RequireRemoteSitePresenceConstraint(site="group2", required_number=2)],
             ),
         )
 
@@ -70,7 +70,7 @@ class TestRemoteRotas:
                 workers_required=1,
                 balance_offset=10,
                 force_as_block=True,
-                constraint=[{"name":"require_remote_site_presence_week", "options":("group2", 1)}],
+                constraints=[RequireRemoteSitePresenceConstraint(site="group2", required_number=1)],
             ),
             SingleShift(
                 sites=("group1", "group2"), name="b", length= 12.5, days=days[3:],
@@ -130,7 +130,7 @@ class TestRemoteRotas:
                 workers_required=1,
                 balance_offset=10,
                 force_as_block=False,
-                constraint=[{"name":"require_remote_site_presence_week", "options":("group2", 1)}],
+                constraints=[RequireRemoteSitePresenceConstraint(site="group2", required_number=1)],
             ),
             SingleShift(
                 sites=("group1", "group2"), name="b", length= 12.5, days=days[3:],