sctest: Run backup/restore tests if exactly one database has schema c…

…hanged

We skip backup/restore tests in declarative schema changer if more than
one databases have had schema changes, because we are doing database
level backup. This should not change any existing test case but rather
skip new ones (to be added in the next commit) like
```
create index idx on t (i);
create database db;
```

Release note: None

pkg/sql/schemachanger/sctest/framework.go

@@ -753,40 +753,42 @@ func maybeGetDatabaseForIDs(
 	t *testing.T, tdb *sqlutils.SQLRunner, ids catalog.DescriptorIDSet,
 ) (dbName string, exists bool) {
 	const q = `
-SELECT
-	name
-FROM
-	system.namespace
-WHERE
-	id
-	IN (
-			SELECT
-				DISTINCT
-				COALESCE(
-					d->'database'->>'id',
-					d->'schema'->>'parentId',
-					d->'type'->>'parentId',
-					d->'function'->>'parentId',
-					d->'table'->>'parentId'
-				)::INT8
-			FROM
-				(
-					SELECT
-						crdb_internal.pb_to_json('desc', descriptor) AS d
-					FROM
-						system.descriptor
-					WHERE
-						id IN (SELECT * FROM ROWS FROM (unnest($1::INT8[])))
-				)
-		)
-`
-	err := tdb.DB.QueryRowContext(context.Background(), q, pq.Array(ids.Ordered())).Scan(&dbName)
-	if errors.Is(err, gosql.ErrNoRows) {
+	SELECT
+		name
+	FROM
+		system.namespace
+	WHERE
+		id
+		IN (
+				SELECT
+					DISTINCT
+					COALESCE(
+						d->'database'->>'id',
+						d->'schema'->>'parentId',
+						d->'type'->>'parentId',
+						d->'function'->>'parentId',
+						d->'table'->>'parentId'
+					)::INT8
+				FROM
+					(
+						SELECT
+							crdb_internal.pb_to_json('desc', descriptor) AS d
+						FROM
+							system.descriptor
+						WHERE
+							id IN (SELECT * FROM ROWS FROM (unnest($1::INT8[])))
+					)
+			)
+	`
+	results := tdb.QueryStr(t, q, pq.Array(ids.Ordered()))
+	if len(results) > 1 {
+		skip.IgnoreLintf(t, "requires all schema changes to happen within one database;"+
+			" get %v: %v", len(results), results)
+	}
+	if len(results) == 0 {
 		return "", false
 	}
-
-	require.NoError(t, err)
-	return dbName, true
+	return results[0][0], true
 }
 
 // withPostCommitPlanAfterSchemaChange