Fix tests for pt-online-schema-change

$ prove -vw --trap --timer t/pt-online-schema-change/
...
time="2023-01-06T17:47:37+03:00" level=info msg="1000 rows inserted"
     0s [====================================================================] 100%
# 1000 rows loaded. Starting tests.
not ok 1 - PT-1455 Successfully altered. Exit status = 0

#   Failed test 'PT-1455 Successfully altered. Exit status = 0'
#   at t/pt-online-schema-change/pt-1455.t line 76.
#          got: '25'
#     expected: '0'
not ok 2 - PT-1455 Got successfully altered message.

#   Failed test 'PT-1455 Got successfully altered message.'
#   at t/pt-online-schema-change/pt-1455.t line 82.
#                   'Found 2 slaves:
# Delly-7390 -> 127.0.0.1:12346
# Delly-7390 -> 127.0.0.1:12347
# Will check slave lag on:
# Delly-7390 -> 127.0.0.1:12346
# Delly-7390 -> 127.0.0.1:12347
# Operation, tries, wait:
#   analyze_table, 10, 1
#   copy_rows, 10, 0.25
#   create_triggers, 10, 1
#   drop_triggers, 10, 1
#   swap_tables, 10, 1
#   update_foreign_keys, 10, 1
# Altering `employees`.`t1`...
# Creating new table...
# Created new table employees._t1_new OK.
# Altering new table...
# Altered `employees`.`_t1_new` OK.
# 2023-01-06T17:47:38 Creating triggers...
# 2023-01-06T17:47:38 Created triggers OK.
# 2023-01-06T17:47:38 Copying approximately 17 rows...
# 2023-01-06T17:47:38 Dropping triggers...
# 2023-01-06T17:47:38 Dropped triggers OK.
# 2023-01-06T17:47:38 Dropping new table...
# 2023-01-06T17:47:38 Dropped new table OK.
# `employees`.`t1` was not altered.
# 	(in cleanup) 2023-01-06T17:47:38 Error copying rows from `employees`.`t1` to `employees`.`_t1_new`: DBD::mysql::db selectrow_hashref failed: Unknown database 'employees' [for Statement "EXPLAIN SELECT * FROM `employees`.`t1` WHERE 1=1"] at /home/sveta/src/percona-toolkit/bin/pt-online-schema-change line 6117.
# 2023-01-06T17:47:38 Error copying rows from `employees`.`t1` to `employees`.`_t1_new`: DBD::mysql::db selectrow_hashref failed: Unknown database 'employees' [for Statement "EXPLAIN SELECT * FROM `employees`.`t1` WHERE 1=1"] at /home/sveta/src/percona-toolkit/bin/pt-online-schema-change line 6117.
# '
#     doesn't match '(?^s:Successfully altered)'
# Resetting replication filters on slave 2
# Resetting replication filters on slave 1
...