bug: fix dns panic during tests

Depending on your router, the dns test panics:

```
$ go19 test -v -race -run TestBatch .
[Jan 15 09:32:30]  INFO coprocess: Disabled feature
=== RUN   TestBatch
panic: Following domain not mocked:localhost.fritz.box.

goroutine 338 [running]:
github.com/TykTechnologies/tyk.(*dnsMockHandler).ServeDNS(0x2922f78, 0x2816a20, 0xc420730400, 0xc4203fa480)
	/Users/ahmet/go/src/github.com/TykTechnologies/tyk/helpers_test.go:539 +0x8cc
github.com/TykTechnologies/tyk/vendor/github.com/miekg/dns.(*Server).serve(0xc4204e6000, 0x280abc0, 0xc42019f3b0, 0x2801cc0, 0x2922f78, 0xc4204fc000, 0x25, 0x200, 0xc42000e030, 0xc4202a05e0, ...)
	/Users/ahmet/go/src/github.com/TykTechnologies/tyk/vendor/github.com/miekg/dns/server.go:558 +0x7a8
created by github.com/TykTechnologies/tyk/vendor/github.com/miekg/dns.(*Server).serveUDP
	/Users/ahmet/go/src/github.com/TykTechnologies/tyk/vendor/github.com/miekg/dns/server.go:514 +0x363
exit status 2
FAIL	github.com/TykTechnologies/tyk	0.346s
```

This PR fixes the issue by automatically pointing to 127.0.0.1 if domain
not in the `domainsToAddresses` map.

helpers_test.go

@@ -513,7 +513,8 @@ func buildAndLoadAPI(apiGens ...func(spec *APISpec)) (specs []*APISpec) {
 	return loadAPI(buildAPI(apiGens...)...)
 }
 
-var domainsToAddresses map[string]string = map[string]string{
+// TODO - Can we just remove this? It doesn't make sense because is dependent on what your local router is.
+var domainsToAddresses = map[string]string{
 	"host1.local.":     "127.0.0.1",
 	"host2.local.":     "127.0.0.1",
 	"host3.local.":     "127.0.0.1",
@@ -522,22 +523,23 @@ var domainsToAddresses map[string]string = map[string]string{
 
 type dnsMockHandler struct{}
 
-func (this *dnsMockHandler) ServeDNS(w dns.ResponseWriter, r *dns.Msg) {
+func (d *dnsMockHandler) ServeDNS(w dns.ResponseWriter, r *dns.Msg) {
 	msg := dns.Msg{}
 	msg.SetReply(r)
 	switch r.Question[0].Qtype {
 	case dns.TypeA:
 		msg.Authoritative = true
 		domain := msg.Question[0].Name
+
 		address, ok := domainsToAddresses[domain]
-		if ok {
-			msg.Answer = append(msg.Answer, &dns.A{
-				Hdr: dns.RR_Header{Name: domain, Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: 60},
-				A:   net.ParseIP(address),
-			})
-		} else {
-			panic("Following domain not mocked:" + domain)
+		if !ok {
+			address = "127.0.0.1"
 		}
+
+		msg.Answer = append(msg.Answer, &dns.A{
+			Hdr: dns.RR_Header{Name: domain, Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: 60},
+			A:   net.ParseIP(address),
+		})
 	}
 	w.WriteMsg(&msg)
 }