tests: relax mqtt warning check

To account for changing error message in Suricata.

tests/mqtt-limit-log-fail/test.yaml

@@ -13,5 +13,5 @@ exit-code: 1
 checks:
   - shell:
         args: |
-          grep "misc: invalid size argument - XXX. Valid size argument should be in the format" stderr | wc -l 
+          grep "invalid size argument" stderr | wc -l 
         expect: 1