CQ: Change the scan test to pad with zeroes instead of random bytes

We don't expect random bytes to be there in the current
version of the message store as we overwrite empty spaces
with zeroes when moving messages around.

deps/rabbit/test/backing_queue_SUITE.erl

@@ -668,8 +668,8 @@ gen_msg_file(Config, Blocks) ->
         {bin, Bin} ->
             Bin;
         {pad, Size} ->
-            %% This might generate false positives although very unlikely.
-            rand:bytes(Size);
+            %% Empty space between messages is expected to be zeroes.
+            <<0:Size/unit:8>>;
         {msg, MsgId, Msg} ->
             Size = 16 + byte_size(Msg),
             [<<Size:64>>, MsgId, Msg, <<255>>]