test: run tests on system level mdadm

test: run tests on system level mdadm

The tests run with MDADM_NO_SYSTEMCTL flag by default, however it has
no effect on udev. In case of external metadata, even if flag is set,
udev will trigger systemd to launch mdmon.

This commit changes test execution level, so the tests are run on system
level mdadm, meaning local build must be installed prior to running
tests.

Add warning that the tests are run on system level mdadm and local
build must be installed first.

Do not call mdadm with "quiet" as it makes it not display critical
messages necessary for debug.

Remove forcing speed_limit and add restoring system speed_limit_max
after test execution.

Signed-off-by: Mateusz Kusiak <mateusz.kusiak@intel.com>
Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>