Fix argument to Builtin.Should Match Regexp call

Change-Id: I4ca109872c38da15828578187958094cb6b9a4fd
diff --git a/libraries/utils.robot b/libraries/utils.robot
index c41e8eb..b345b0f 100755
--- a/libraries/utils.robot
+++ b/libraries/utils.robot
@@ -1310,7 +1310,7 @@
         Execute Remote Command    sudo pkill mausezahn
         ...    ${src_ip}    ${src_user}    ${src_pass}    ${src_container_type}    ${src_container_name}
         Run Keyword If    "${tcpdump_filter}"=="tcp"
-        ...    Should Match Regexp    , p ${pbit},
+        ...    Should Match Regexp    ${output}    , p ${pbit},
     END
 
 Determine Number Of ONU