Changing the mariabackup history table from PERCONA_SCHEMA.xtrabackup_history
to mysql.mariabackup_history.

Additionally, extending xb_history.test for better coverage:

- Recording the fact that the history table is created during
  "mariabackup --history" invocation when it does not exist.

- Recording the history table structure (adding SHOW CREATE TABLE)

- Recording how --history vs --history=foo affect the "name" column
  of the history table.

- Recording the fact that two consequent executions of
  "mariabackup --history[=foo]" insert into the history table
  incrementally, without truncating it on every execution.