Alerting: Validate contact point configuration during migration to Unified Alerting (#40717)

* Alerting: Validate contact point configuration during the migration

This minimises the chances of generating broken configuration as part of the migration. Originally, we wanted to generate it and not produce a hard stop in Grafana but this strategy has the chance to avoid delivering notifications for our users.

We now think it's better to hard stop the migration and let the user take care of resolving the configuration manually.

pkg/services/sqlstore/migrations/ualert/testing.go

@@ -0,0 +1,23 @@
+package ualert
+
+import (
+	"testing"
+
+	"github.com/grafana/grafana/pkg/infra/log"
+	"github.com/grafana/grafana/pkg/services/sqlstore/migrator"
+)
+
+// newTestMigration generates an empty migration to use in tests.
+func newTestMigration(t *testing.T) *migration {
+	t.Helper()
+
+	return &migration{
+		mg: &migrator.Migrator{
+
+			Logger: log.New("test"),
+		},
+		migratedChannelsPerOrg:    make(map[int64]map[*notificationChannel]struct{}),
+		portedChannelGroupsPerOrg: make(map[int64]map[string]string),
+		seenChannelUIDs:           make(map[string]struct{}),
+	}
+}