Add a replication MTR test cloning the slave with mariadb-backup

The test should start as a usual replication test:

• run some statements on the master
• make sure they are propagated to the slave side

Then, the test should emulate cloning data from the current slave to a new virgin slave.

This sequence on the slave should do:

• mariadb-backup --backup
• shutdown the server
• drop the entire data directory
• mariadb-backup --prepare
• mariadb-backup --copy-back
• restart the server
• configure replication using CHANGE MASTER to set user, host, port
• configure replication using xtrabackup_slave_info, to set log file name and log position
• restart replication using START SLAVE

In parallel with these steps on the slave, the master should run some more transactions at different points in time:

• while the "old" slave is running
• while the slave is shut down
• while the "new" slave is running

At the end, the test should check that all master transactions are reflected on the "new" slave.