mdadm: drop from PTESTS_SLOW

mdadm: drop from PTESTS_SLOW

There are two primary reasons for this:

1. mdadm test suite tends to work like this

do_action
sleep arbitrary_amount
check_result

This is unreliable, and arbitrary_amount may or may not be enough.
I have confirmed this by increasing the amount, and seeing more tests
pass than before.

2. The test suite aborts half way through because one of the mdadm binaries
segfaults. This indicates that upstream isn't actually running the suite
anymore, and in this situation we shouldn't be relying on it either.

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>