functional: test sample clients file is valid

Also tests that it's being loaded properly (which is not the case in
NewClientManagerFromClients, which will be removed in subsequent commit)

functional/config/config_sample_test.go

+package config
+
+import (
+	"os"
+	"testing"
+
+	"github.com/coreos/dex/client"
+	"github.com/coreos/dex/client/manager"
+	"github.com/coreos/dex/db"
+)
+
+const (
+	clientsFile = "../../static/fixtures/clients.json.sample"
+)
+
+// TestClientSample makes sure that the clients.json.sample file is valid and can be loaded properly.
+func TestClientSample(t *testing.T) {
+	f, err := os.Open(clientsFile)
+	if err != nil {
+		t.Fatalf("could not open file %q: %v", clientsFile, err)
+	}
+	defer f.Close()
+
+	clients, err := client.ClientsFromReader(f)
+	if err != nil {
+		t.Fatalf("Error loading Clients: %v", err)
+	}
+
+	memDB := db.NewMemDB()
+	repo := db.NewClientRepo(memDB)
+	for _, c := range clients {
+		repo.New(nil, c)
+	}
+	mgr := manager.NewClientManager(repo, db.TransactionFactory(memDB), manager.ManagerOptions{})
+
+	for i, c := range clients {
+		ok, err := mgr.Authenticate(c.Credentials)
+		if !ok {
+			t.Errorf("case %d: couldn't authenticate", i)
+		}
+		if err != nil {
+			t.Errorf("case %d: error authenticating: %v", i, err)
+		}
+	}
+
+}

test-functional

@@ -4,3 +4,4 @@ source ./env
 
 go test $@ github.com/coreos/dex/functional
 go test $@ github.com/coreos/dex/functional/repo
+go test $@ github.com/coreos/dex/functional/config