test: Add TLS tracking integration test

This patch adds a new test, which verifies the TLS tracking.

Because we need to simulate SPF records, and Go does not support fully
intercepting DNS lookups yet, this test relies on dnsmasq to provide a
DNS resolver.

In the future, once Go supports DNS lookup interception, we can get rid
of this additional dependency.

test/t-14-tls_tracking/dnsmasq.conf

@@ -0,0 +1,24 @@
+# Configuration for dnsmasq, for testing purposes.
+
+interface=lo
+port=9053
+no-resolv
+no-poll
+no-hosts
+
+log-queries
+
+# Note we need both ipv4 and ipv6 A record because some test environments may
+# not support one or the other.
+
+# srv-a zone
+address=/srv-a/::1
+address=/srv-a/127.0.0.1
+mx-host=srv-a,srv-a,10
+txt-record=srv-a,"v=spf1 a"
+
+# srv-b zone
+address=/srv-b/::1
+address=/srv-b/127.0.0.1
+mx-host=srv-b,srv-b,10
+txt-record=srv-b,"v=spf1 a"