proxy_test: Add helper to read all from a reply
proxy_test: Add helper to read all from a reply

Prep for testing `GetConfig`.

Signed-off-by: Colin Walters <694ce38231ed622bd1593ce4ec9aa85c9ddb0a78@verbum.org>

mtrmac/skopeo,mtrmac/skopeo,mtrmac/skopeo