Improve frameworktestutil usability with complex error messages

kyaml/fn/framework/testdata/validation/error/errors.txt

@@ -0,0 +1,5 @@
+JavaSpringBoot had 4 errors:
+\[\d\] replicas must be less than 10
+\[\d\] name is required
+\[\d\] image should not have latest tag
+\[\d\] domain must be a subdomain of example.com

kyaml/fn/framework/testdata/validation/error/input.yaml

@@ -0,0 +1,14 @@
+# Copyright 2021 The Kubernetes Authors.
+# SPDX-License-Identifier: Apache-2.0
+
+kind: ResourceList
+apiVersion: config.kubernetes.io/v1alpha1
+functionConfig:
+  apiVersion: example.com/v1alpha1
+  kind: JavaSpringBoot
+  metadata:
+    name: ""
+  spec:
+    replicas: 1000
+    image: foo:latest
+    domain: bad