add automated test for quickstart

with tls with acme (with pebble, a small acme server for testing), and with
pregenerated keys/certs.

the two mox instances are configured on their own domain. we launch a separate
test container that connects to the first, submits a message for delivery to
the second. we check if the message is delivered with an imap connection and
the idle command.

testdata/quickstart/Dockerfile.test

@@ -0,0 +1,4 @@
+FROM golang:1-alpine AS build
+WORKDIR /mox
+RUN apk add make bind-tools bash unbound curl
+env GOPROXY=off