Code review comment for lp:~gl-az/percona-xtrabackup/2.1-bug1197644

Using global files (under /tmp) in tests is a bad idea, because 1) there may be a conflict with other concurrently running tests and 2) they won't be removed if the test fails.

If you create them under $TEST_VAR_ROOT they will be unique for each test and will be automatically removed by the test suite if the test does not clean up itself.