tests: add test that DDF marks missing devices as failed on assembly.

If we assemble a newly-degraded array, the missing devices must be marked
as 'failed' so we don't expect them in future.

Signed-off-by: NeilBrown <neilb@suse.de>

tests/10ddf-sudden-degraded

@@ -0,0 +1,17 @@
+#
+# An array is assembled with one device missing.
+# The other device must be marked as Failed in metadata
+
+. tests/env-ddf-template
+
+mdadm -CR $container -e ddf -n 2 $dev8 $dev9
+ddf_check container 2
+
+mdadm -CR $member1 -n 2 -l1 $dev8 $dev9
+mdadm --wait $member1
+mdadm -Ss
+
+mdadm -I $dev8
+mdadm -R $container
+mkfs $member1
+mdadm --examine $dev8 | grep 'Failed' || exit 1