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.go

@@ -12,13 +12,13 @@ type queryEvaluator struct {
 	eval eval.ConditionEvaluator
 }
 
-func (d *queryEvaluator) Eval(ctx context.Context, from, to time.Time, interval time.Duration, callback callbackFunc) error {
-	for now := from; now.Before(to); now = now.Add(interval) {
+func (d *queryEvaluator) Eval(ctx context.Context, from time.Time, interval time.Duration, evaluations int, callback callbackFunc) error {
+	for idx, now := 0, from; idx < evaluations; idx, now = idx+1, now.Add(interval) {
 		results, err := d.eval.Evaluate(ctx, now)
 		if err != nil {
 			return err
 		}
-		err = callback(now, results)
+		err = callback(idx, now, results)
 		if err != nil {
 			return err
 		}