CQ: Fix flakes in the store file scan test

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.

We also don't expect messages to be false flagged when
the broker is running because it checks for message
validity in the index. Therefore make sure message bodies
in the tests don't contain byte 255.

(cherry picked from commit db50739)

deps/rabbit/test/backing_queue_SUITE.erl

@@ -655,9 +655,11 @@ gen_msg() ->
     gen_msg(1024 * 1024).
 
 gen_msg(MaxSize) ->
-    %% This might generate false positives but very rarely
-    %% so we don't do anything to prevent them.
-    rand:bytes(rand:uniform(MaxSize)).
+    Bytes = rand:bytes(rand:uniform(MaxSize)),
+    %% We remove 255 to avoid false positives. In a running
+    %% rabbit node we will not get false positives because
+    %% we also check messages against the index.
+    << <<case B of 255 -> 254; _ -> B end>> || <<B>> <= Bytes >>.
 
 gen_msg_file(Config, Blocks) ->
     PrivDir = ?config(priv_dir, Config),
@@ -667,8 +669,8 @@ 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>>]