[1.8.x] Fixed #26116 -- Corrected schema's test_alter_implicit_id_to_explicit.

AUTOINCREMENT is dropped converting an AutoField to IntegerField
which isn't the point of this test. MySQL would warn or error about
this.

Backport of b49cc86643 from master

tests/schema/tests.py

@@ -722,7 +722,7 @@ class SchemaTests(TransactionTestCase):
             editor.create_model(Author)
 
         old_field = Author._meta.get_field("id")
-        new_field = IntegerField(primary_key=True)
+        new_field = AutoField(primary_key=True)
         new_field.set_attributes_from_name("id")
         new_field.model = Author
         with connection.schema_editor() as editor:
@@ -730,6 +730,7 @@ class SchemaTests(TransactionTestCase):
         # This will fail if DROP DEFAULT is inadvertently executed on this
         # field which drops the id sequence, at least on PostgreSQL.
         Author.objects.create(name='Foo')
+        Author.objects.create(name='Bar')
 
     def test_alter_int_pk_to_autofield_pk(self):
         """