CI: Output a summary using GITHUB_SUMMARY

Uses teststat to parse the go test json and output markdown which will
be posted as a summary to the github action run.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

script/setup/install-teststat

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+
+#   Copyright The containerd Authors.
+
+#   Licensed under the Apache License, Version 2.0 (the "License");
+#   you may not use this file except in compliance with the License.
+#   You may obtain a copy of the License at
+
+#       http://www.apache.org/licenses/LICENSE-2.0
+
+#   Unless required by applicable law or agreed to in writing, software
+#   distributed under the License is distributed on an "AS IS" BASIS,
+#   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#   See the License for the specific language governing permissions and
+#   limitations under the License.
+
+GO111MODULE=on go install github.com/vearutop/teststat@v0.1.3