test: add external compilation test (#803)

Pull down example middleware and compile it in (this should compile)
then check if dns.example is included in the list of plugins
(middlewares)

test/external_test.go

+package test
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+	"testing"
+)
+
+// Go get external example middleware, compile it into CoreDNS
+// and check if it is really there, but running coredns -plugins.
+
+func TestExternalMiddlewareCompile(t *testing.T) {
+	if err := addExampleMiddleware(); err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err := run(t, goGet); err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err := run(t, goGen); err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err := run(t, goBuild); err != nil {
+		t.Fatal(err)
+	}
+
+	out, err := run(t, coredns)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !strings.Contains(string(out), "dns.example") {
+		t.Fatal("dns.example middleware should be there")
+	}
+}
+
+func run(t *testing.T, c *exec.Cmd) ([]byte, error) {
+	c.Dir = ".."
+	out, err := c.Output()
+	if err != nil {
+		return nil, fmt.Errorf("run: failed to run %s %s: %q", c.Args[0], c.Args[1], err)
+	}
+	return out, nil
+
+}
+
+func addExampleMiddleware() error {
+	f, err := os.OpenFile("../middleware.cfg", os.O_APPEND|os.O_WRONLY, os.ModeAppend)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	_, err = f.WriteString(example)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+var (
+	goBuild = exec.Command("go", "build")
+	goGen   = exec.Command("go", "generate")
+	goGet   = exec.Command("go", "get", "github.com/coredns/example")
+	coredns = exec.Command("./coredns", "-plugins")
+)
+
+const example = "1001:example:github.com/coredns/example"