Fix basic income child age bug (#775)

* Add failing unit test

* Fix test

* Versioning

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: patch
+  changes:
+    fixed:
+    - Bug causing child minimum basic income ages to function incorrectly when the adult UBI is nonzero.

policyengine_uk/tests/policy/reforms/parametric/basic_income/bi_maximum.yaml

@@ -0,0 +1,10 @@
+- name: Minimum child age functions correctly when adult is non zero.
+  period: 2021
+  input:
+    gov.contrib.ubi_center.basic_income.amount.child_min_age: 16
+    gov.contrib.ubi_center.basic_income.amount.by_age.working_age: 1
+    people:
+      child:
+        age: 14
+  output:
+    basic_income: 0

policyengine_uk/variables/contrib/ubi_center/basic_income/bi_maximum.py

@@ -16,9 +16,13 @@ def formula(person, period, parameters):
         is_child_for_bi = (age < bi.amount.adult_age) * (
             age >= bi.amount.child_min_age
         )
+        is_adult_for_bi = (age >= bi.amount.adult_age) * ~is_senior_for_bi
         weekly_amount_by_age = select(
-            [is_child_for_bi, is_senior_for_bi],
-            [bi.amount.by_age.child, bi.amount.by_age.senior],
-            default=bi.amount.by_age.working_age,
+            [is_child_for_bi, is_senior_for_bi, is_adult_for_bi],
+            [
+                bi.amount.by_age.child,
+                bi.amount.by_age.senior,
+                bi.amount.by_age.working_age,
+            ],
         )
         return (weekly_flat_amount + weekly_amount_by_age) * WEEKS_IN_YEAR