Fix a flaky test by not depending on Google (#2565)

TestLookupAutoPathErratic sometimes fail on TravisCI saying below

```
=== RUN   TestLookupAutoPathErratic
--- FAIL: TestLookupAutoPathErratic (8.30s)
    erratic_autopath_test.go:39: Test 0, failed to sent query: "read udp [::1]:39758->[::1]:56643: i/o timeout"
FAIL
```

The failure happens when Google replies slowly.
This PR changes to not use Google but run CoreDNS locally and proxy to the server.

---

Because the failure depends on Google, sometimes it happens frequently but sometimes it doesn't happen.
I hope following test help you reproduce it.

```
func TestLookupAutoPathErratic2(t *testing.T) {
	for i := 0; i < 200; i++ {
		TestLookupAutoPathErratic(t)
	}
}
```

or I can reproduce it by changing proxy to other DNS like 1.1.1.1 instead of 8.8.8.8 too

test/erratic_autopath_test.go

@@ -1,16 +1,53 @@
 package test
 
 import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/miekg/dns"
 )
 
+func setupProxyTargetCoreDNS(t *testing.T, fn func(string)) {
+	tmpdir, err := ioutil.TempDir(os.TempDir(), "coredns")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Remove(tmpdir)
+
+	content := `
+example.org. IN	SOA sns.dns.icann.org. noc.dns.icann.org. 1 3600 3600 3600 3600
+
+google.com. IN SOA ns1.google.com. dns-admin.google.com. 1 3600 3600 3600 3600
+google.com. IN A 172.217.25.110
+`
+
+	path := filepath.Join(tmpdir, "file")
+	if err = ioutil.WriteFile(path, []byte(content), 0644); err != nil {
+		t.Fatalf("Could not write to temp file: %s", err)
+	}
+	defer os.Remove(path)
+
+	corefile := `.:0 {
+	file ` + path + `
+}
+`
+	i, udp, _, err := CoreDNSServerAndPorts(corefile)
+	if err != nil {
+		t.Fatalf("Could not get proxy target CoreDNS serving instance: %s", err)
+	}
+	defer i.Stop()
+
+	fn(udp)
+}
+
 func TestLookupAutoPathErratic(t *testing.T) {
+	setupProxyTargetCoreDNS(t, func(proxyPath string) {
 		corefile := `.:0 {
 		erratic
 		autopath @erratic
-	proxy . 8.8.8.8:53
+		proxy . ` + proxyPath + `
 		debug
 		}
 `
@@ -48,12 +85,14 @@ func TestLookupAutoPathErratic(t *testing.T) {
 				t.Fatalf("Test %d, expected answer type %d, got %d", i, tc.expectedType, x)
 			}
 		}
+	})
 }
 
 func TestAutoPathErraticNotLoaded(t *testing.T) {
+	setupProxyTargetCoreDNS(t, func(proxyPath string) {
 		corefile := `.:0 {
 	autopath @erratic
-	proxy . 8.8.8.8:53
+	proxy . ` + proxyPath + `
 	debug
     }
 `
@@ -77,4 +116,5 @@ func TestAutoPathErraticNotLoaded(t *testing.T) {
 		if r.Rcode != dns.RcodeNameError {
 			t.Fatalf("Expected NXDOMAIN, got %d", r.Rcode)
 		}
+	})
 }