mw/file: add include test (#960)

$INCLUDE was fixed in miekg/dns, create tests in CoreDNS so that we know
we have the latest version.

Fixes #880

middleware/file/include_test.go

+package file
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/coredns/coredns/middleware/test"
+)
+
+// Make sure the external miekg/dns dependency is up to date
+
+func TestInclude(t *testing.T) {
+
+	name, rm, err := test.TempFile(".", "foo\tIN\tA\t127.0.0.1\n")
+	if err != nil {
+		t.Fatalf("Unable to create tmpfile %q: %s", name, err)
+	}
+	defer rm()
+
+	zone := `$ORIGIN example.org.
+@ IN SOA sns.dns.icann.org. noc.dns.icann.org. 2017042766 7200 3600 1209600 3600
+$INCLUDE ` + name + "\n"
+
+	z, err := Parse(strings.NewReader(zone), "example.org.", "test", 0)
+	if err != nil {
+		t.Errorf("Unable to parse zone %q: %s", "example.org.", err)
+	}
+
+	if _, ok := z.Search("foo.example.org."); !ok {
+		t.Errorf("Failed to find %q in parsed zone", "foo.example.org.")
+	}
+}