Fixing buildbot failures on mariabackup.aria_log_dir_path_rel.

The problem was that directory_exists() was called with the
relative aria_log_dir_path value, while the current directory
in mariadb-backup is not necessarily equal to datadir when MTR is running.

Fix:

- Moving building the absolute path un level upper:
  from the function copy_back_aria_logs() to the function copy_back().
- Passing the built absolute path to both directory_exists() and
  copy_back_aria_logs() as a parameter.