[5.0.x] Refs #35194 -- Adjusted a generated field test to work on Postgres 15.6+.

Postgres >= 12.18, 13.14, 14.11, 15.6, 16.2 changed the way the immutability
of generated and default expressions is detected in postgres/postgres@743ddaf.

The adjusted test semantic is presereved by switching from __icontains to
__contains as both make use of a `%` literal which requires proper escaping.

Refs #35336.

Thanks bcail for the report.

Backport of 73b62a2126 from main.

tests/schema/tests.py

@@ -919,7 +919,7 @@ class SchemaTests(TransactionTestCase):
             editor.create_model(GeneratedFieldContainsModel)
 
         field = GeneratedField(
-            expression=Q(text__icontains="FOO"),
+            expression=Q(text__contains="foo"),
             db_persist=True,
             output_field=BooleanField(),
         )