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/pebble-config.json

@@ -0,0 +1,12 @@
+{
+	"pebble": {
+		"listenAddress": "0.0.0.0:14000",
+		"managementListenAddress": "0.0.0.0:15000",
+		"certificate": "/quickstart/tls/acmepebble.pem",
+		"privateKey": "/quickstart/tls/acmepebble-key.pem",
+		"httpPort": 80,
+		"tlsPort": 443,
+		"ocspResponderURL": "",
+		"externalAccountBindingRequired": false
+	}
+}