Alerting: Fix panic in backtesting API when the testing interval is not times of evaluation interval (#68727)

* add test for the bug
* update backtesting evaluators to accept a number of evaluations instead of `to` to have control over the number evaluations in one place

pkg/services/ngalert/backtesting/eval_query_test.go

@@ -7,6 +7,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/mock"
 	"github.com/stretchr/testify/require"
 
@@ -18,8 +19,7 @@ func TestQueryEvaluator_Eval(t *testing.T) {
 	ctx := context.Background()
 	interval := time.Duration(rand.Int63n(9)+1) * time.Second
 	times := rand.Intn(11) + 5
-	to := time.Now()
-	from := to.Add(-time.Duration(times) * interval)
+	from := time.Now().Add(-time.Duration(times) * interval)
 
 	t.Run("should evaluate query", func(t *testing.T) {
 		m := &eval_mocks.ConditionEvaluatorMock{}
@@ -29,15 +29,20 @@ func TestQueryEvaluator_Eval(t *testing.T) {
 			eval: m,
 		}
 
-		intervals := make([]time.Time, 0, times)
+		intervals := make([]time.Time, times)
 
-		err := evaluator.Eval(ctx, from, to, interval, func(now time.Time, results eval.Results) error {
-			intervals = append(intervals, now)
+		err := evaluator.Eval(ctx, from, interval, times, func(idx int, now time.Time, results eval.Results) error {
+			intervals[idx] = now
 			return nil
 		})
 		require.NoError(t, err)
 		require.Len(t, intervals, times)
 
+		expected := from
+		for idx, actual := range intervals {
+			assert.Equalf(t, expected, actual, "item at index %d is not times of interval %v", idx, interval)
+			expected = expected.Add(interval)
+		}
 		m.AssertNumberOfCalls(t, "Evaluate", times)
 		for _, now := range intervals {
 			m.AssertCalled(t, "Evaluate", ctx, now)
@@ -57,7 +62,7 @@ func TestQueryEvaluator_Eval(t *testing.T) {
 
 			intervals := make([]time.Time, 0, times)
 
-			err := evaluator.Eval(ctx, from, to, interval, func(now time.Time, results eval.Results) error {
+			err := evaluator.Eval(ctx, from, interval, times, func(idx int, now time.Time, results eval.Results) error {
 				intervals = append(intervals, now)
 				return nil
 			})
@@ -76,7 +81,7 @@ func TestQueryEvaluator_Eval(t *testing.T) {
 
 			intervals := make([]time.Time, 0, times)
 
-			err := evaluator.Eval(ctx, from, to, interval, func(now time.Time, results eval.Results) error {
+			err := evaluator.Eval(ctx, from, interval, times, func(idx int, now time.Time, results eval.Results) error {
 				if len(intervals) > 3 {
 					return expectedError
 				}