iscsi migration test: Extend the execution time of fio.

When we conduct the ip migration test (i.e., switch the iSCSI target
process from to first process to
second). This time may exceed the 5s in some cases, so
FIO process will exit with error and make the test fail.
Because The total FIO execution time is 10s, and we sleep 5s
in the script. So propose the following approach:

1 Change the sleep from 5s to 3s, we do not need to sleep
it too long for FIO.
2 Extend the FIO execution time from 10s to 12s.

Signed-off-by: Ziye Yang <ziye.yang@intel.com>
Change-Id: I07d488c6e8a94733534acc1acd2ba8778c4d2eb3
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/2801


Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>