test926: Corrected unknown user reply from commit 0d735c29f9

The error code should not be sent as data as it isn't passed onto the
client as body data, so cannot be compared in the test suite against
expected data.

tests/data/test926

@@ -9,9 +9,9 @@ VRFY
 #
 # Server-side
 <reply>
-<data>
-550 Unknown user
-</data>
+<servercmd>
+REPLY VRFY 550 Unknown user
+</servercmd>
 </reply>
 
 #