Fix sysbench fileio test

[leivos-unikie]

Used #191 as the starting point.

Notes:

• Each Execute Command keyword run in a new shell. So doing cd on one Execute Command line does not affect the working directory of another Execute Command line. However, with Write it remains in the same shell.
• Had to Read Until "Test finished.". Read Until Prompt didn't work for some reason.
• Had to increase timeout temporarily for Read Until. For now I set it to 15min (and then back to 30s), should be enough as on my local test runs the fileio test took 4.5 minutes.