Expressions: Fixes the issue showing expressions editor (#62510)

* Use suggested value for uid

* update the snapshot

* use __expr__

* replace all -100 with __expr__

* update snapshot

* more changes

* revert redundant change

* Use expr.DatasourceUID where it's possible

* generate files

pkg/tests/api/alerting/testing.go

@@ -10,6 +10,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/grafana/grafana/pkg/expr"
 	"github.com/prometheus/common/model"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -105,7 +106,7 @@ func alertRuleGen() func() apimodels.PostableExtendedRuleNode {
 							From: ngmodels.Duration(time.Duration(5) * time.Hour),
 							To:   ngmodels.Duration(time.Duration(3) * time.Hour),
 						},
-						DatasourceUID: "-100",
+						DatasourceUID: expr.DatasourceUID,
 						Model: json.RawMessage(`{
 								"type": "math",
 								"expression": "2 + 3 > 1"