Have calls to sqlite3_backup_init() fail if there is already a read or read-write transaction open on the destination database.

FossilOrigin-Name: 169b5505498c0a7ee2b5dbb2ba13c41dfaa7c62f

test/backup.test

@@ -217,6 +217,7 @@ foreach nPagePerStep {1 200} {
           INSERT INTO ${file_dest}.t1 VALUES(1, randstr(1000,1000))
         " $db_dest
       }
+      execsql COMMIT $db_dest
     }
   
     # Backup the source database.