logging_SUITE: Ensure the exchange was declared in logging_to_exchange_works

[Why]
We were running the check to make sure the exchange was declared, but we
didn't verify the result of that check. The testcase would still fail
later but if we verify its existence early, the testcase can fail early
too.

(cherry picked from commit 3cc3a38)
(cherry picked from commit b197867)

Author: dumbbell

deps/rabbit/test/logging_SUITE.erl

@@ -1035,17 +1035,18 @@ logging_to_exchange_works(Config) ->
        UpgradeFileHandler),
 
     %% Wait for the expected exchange to be automatically declared.
-    lists:any(
-      fun(_) ->
-              Ret = rabbit_ct_broker_helpers:rpc(
-                      Config, 0,
-                      rabbit_exchange, lookup, [Exchange]),
-              case Ret of
-                  {ok, _} -> true;
-                  _       -> timer:sleep(500),
-                             false
-              end
-      end, lists:seq(1, 20)),
+    ?assert(
+       lists:any(
+         fun(_) ->
+                 Ret = rabbit_ct_broker_helpers:rpc(
+                         Config, 0,
+                         rabbit_exchange, lookup, [Exchange]),
+                 case Ret of
+                     {ok, _} -> true;
+                     _       -> timer:sleep(500),
+                                false
+                 end
+         end, lists:seq(1, 20))),
 
     %% Declare a queue to collect all logged messages.
     {Conn, Chan} = rabbit_ct_client_helpers:open_connection_and_channel(