mdadm/test: Add '--disks=' to support testing phsical devices

mdadm/test: Add '--disks=' to support testing phsical devices

If test mode has set as '--dev=disk', then users can specify
the argument of "--disks" to test a bunch of physical devices.
For example: ./test --dev=disk --disks=/dev/sda{2..15}
could execute all test cases on physical devices.

Currently, the --dev=disk mode would confilct with testdev()
in current test cases, thus ignore testing testdev() if has
set --dev=disk mode.

Signed-off-by: Zhilong Liu <zlliu@suse.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>