Alerting: Split Scheduler and AlertRouter tests (#52416)

* move fake FakeExternalAlertmanager to sender package
* move tests from scheduler to router
* update alerts router to have all fields private
* update scheduler tests to use sender mock

pkg/services/ngalert/sender/testing.go

@@ -0,0 +1,88 @@
+package sender
+
+import (
+	"encoding/json"
+	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
+	"sync"
+	"testing"
+
+	amv2 "github.com/prometheus/alertmanager/api/v2/models"
+	"github.com/prometheus/common/model"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+type FakeExternalAlertmanager struct {
+	t      *testing.T
+	mtx    sync.Mutex
+	alerts amv2.PostableAlerts
+	Server *httptest.Server
+}
+
+func NewFakeExternalAlertmanager(t *testing.T) *FakeExternalAlertmanager {
+	t.Helper()
+
+	am := &FakeExternalAlertmanager{
+		t:      t,
+		alerts: amv2.PostableAlerts{},
+	}
+	am.Server = httptest.NewServer(http.HandlerFunc(am.Handler()))
+
+	return am
+}
+
+func (am *FakeExternalAlertmanager) URL() string {
+	return am.Server.URL
+}
+
+func (am *FakeExternalAlertmanager) AlertNamesCompare(expected []string) bool {
+	n := []string{}
+	alerts := am.Alerts()
+
+	if len(expected) != len(alerts) {
+		return false
+	}
+
+	for _, a := range am.Alerts() {
+		for k, v := range a.Alert.Labels {
+			if k == model.AlertNameLabel {
+				n = append(n, v)
+			}
+		}
+	}
+
+	return assert.ObjectsAreEqual(expected, n)
+}
+
+func (am *FakeExternalAlertmanager) AlertsCount() int {
+	am.mtx.Lock()
+	defer am.mtx.Unlock()
+
+	return len(am.alerts)
+}
+
+func (am *FakeExternalAlertmanager) Alerts() amv2.PostableAlerts {
+	am.mtx.Lock()
+	defer am.mtx.Unlock()
+	return am.alerts
+}
+
+func (am *FakeExternalAlertmanager) Handler() func(w http.ResponseWriter, r *http.Request) {
+	return func(w http.ResponseWriter, r *http.Request) {
+		b, err := ioutil.ReadAll(r.Body)
+		require.NoError(am.t, err)
+
+		a := amv2.PostableAlerts{}
+		require.NoError(am.t, json.Unmarshal(b, &a))
+
+		am.mtx.Lock()
+		am.alerts = append(am.alerts, a...)
+		am.mtx.Unlock()
+	}
+}
+
+func (am *FakeExternalAlertmanager) Close() {
+	am.Server.Close()
+}