file_io_test: Simplify a test with memory::with_allocation_failures
file_io_test: Simplify a test with memory::with_allocation_failures

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200804143351.14211-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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