Minor fix to a unit test: was not properly configure mapper in alt case

Author: cowtowncoder

src/test/java/com/fasterxml/jackson/databind/jsontype/vld/AnnotatedPolymorphicValidationTest.java

@@ -76,6 +76,7 @@ public void testPolymorphicWithUnsafeBaseType() throws IOException
 
         // but may with proper validator
         ObjectMapper customMapper = JsonMapper.builder()
+                .enable(MapperFeature.BLOCK_UNSAFE_POLYMORPHIC_BASE_TYPES)
                 .polymorphicTypeValidator(new NumbersAreOkValidator())
                 .build();