io_tester: Add option to reuse test files
io_tester: Add option to reuse test files

When io-tester is run several times in a row it makes sense
not to remove and create files from scratch. This is a huuuge
and absolitely safe time saving for large files and read
workloads. Write ones should use it with care, but given long
enough testing time it's also safe.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/seastar,scylladb/seastar,cloudius-systems/seastar,cloudius-systems/seastar,syuu1228/seastar,cloudius-systems/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,syuu1228/seastar