Silence dd invocation in all.sh

It brings no value and distracts us from the actual content.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 0a9d806..06d9c5c 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -1886,7 +1886,7 @@
     # Unconditionally create a seedfile that's sufficiently long.
     # Do this before each component, because a previous component may
     # have messed it up or shortened it.
-    dd if=/dev/urandom of=./tests/seedfile bs=64 count=1
+    dd if=/dev/urandom of=./tests/seedfile bs=64 count=1 >/dev/null 2>&1
 
     # Run the component code.
     "$@"