message: compile warning in testcases

Thread functions (non-void) were missing a return.

Change-Id: Ib7804d92f79b96428285058101e64dceea7ad66e
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/test/message/append.cpp b/test/message/append.cpp
index 84dcc28..fcf3dd9 100644
--- a/test/message/append.cpp
+++ b/test/message/append.cpp
@@ -63,6 +63,8 @@
             break;
         }
     }
+
+    return nullptr;
 }
 
 auto newMethodCall__test(sdbusplus::bus::bus& b)