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/config.json

@@ -0,0 +1,17 @@
+{
+	"signing": {
+		"default": {
+			"expiry": "262800h"
+		},
+		"profiles": {
+			"server": {
+				"expiry": "262800h",
+				"usages": [
+					"signing",
+					"key encipherment",
+					"server auth"
+				]
+			}
+		}
+	}
+}