PT-2340 - Support MySQL 8.4

- Removed lib/Percona/Test.pm, lib/Safeguards.pm, t/lib/Safeguards.t, because they are not used anymore
- Removed word "slave" from lib

t/lib/TableSyncer.t

@@ -1004,7 +1004,7 @@ my $output = '';
             },
          );
       },
-      qr/Slave did not catch up to its source after 2 attempts of waiting 60/,
+      qr/Replica did not catch up to its source after 2 attempts of waiting 60/,
       "Retries wait"
    );
 }