Speed up XtraBackup test suite

The XtraBackup test suite execution time is growing. Every test adds about 2-3 minutes, and it can easily become 5-10 minutes on slow Jenkins slaves.

We should look into optimizing the test suite. A large part of tests execution time is spent on activities that are not related to tests itself. Like starting up the server (we can tune InnoDB to make it faster), and loading up the sakila database, which is rather large. This can be optimized by pre-creating the data directory with the sakila databases, and then starting the server on that directory, rather than loading it from SQL for each test.

I think with a few simple optimizations like the above we can cut down the test suite execution time significantly.