mdadm/tests: test enhance

mdadm/tests: test enhance

There are two changes.
First, if md module is not loaded, it gives error when reading
speed_limit_max. So read the value after loading md module which
is done in do_setup

Second, sometimes the test reports error sync action doesn't
happen. But dmesg shows sync action is done. So limit the sync
speed before test. It doesn't affect the test run time. Because
check wait sets the max speed before waiting sync action. And
recording speed_limit_max/min in do_setup.

Fixes: 4c12714d1ca0 ('test: run tests on system level mdadm')
Signed-off-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>