fn: add tester image (#609)

* fn: add fn-test-utils image

New tester image that uses go-fdk for advanced test scenarios.
Right now, this is an enhanced 'hello/sleeper' rolled into one
that echos the received headers/env to allow writing test cases.

release.sh

@@ -45,6 +45,9 @@ docker tag $user/$image:latest $user/$image_deprecated:latest
 docker push $user/$image_deprecated:$version
 docker push $user/$image_deprecated:latest
 
+# release test utils docker image
+(cd images/fn-test-utils && ./release.sh)
+
 cd fnlb
 ./release.sh
 cd ..