Strengthen highest_education test to assert specific enum values present

Author: nikhilwoodruff

policyengine_uk_data/tests/test_highest_education.py

@@ -1,3 +1,5 @@
 def test_highest_education(baseline):
     values = baseline.calculate("highest_education", period=2025)
-    assert len(values) > 0
+    assert "Tertiary" in set(values)
+    assert "Lower Secondary" in set(values)
+    assert "Upper Secondary" in set(values)