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/tls/cfssl-ca-csr.json

@@ -0,0 +1,10 @@
+{
+	"CN": "mox test ca",
+	"key": {
+		"algo": "ecdsa",
+		"size": 256
+	},
+	"ca": {
+		"expiry": "262800h"
+	}
+}