Is there a test string like GTUBE specifically for Message Sniffer? For testing, I want to trigger only Message Sniffer and not other subsystems in my test stack.

Yes. The following test strings will produce specific result codes when found in any message.

RuleID  Test String----------------------

1063388 *SNF-TEST-CODE-63-znSnhtAuXbjaZ5* 1063386 *SNF-TEST-CODE-62-MzwyFiYq1GwIEs* 1063383 *SNF-TEST-CODE-61-IBAC9dooqAI6dJ* 1063381 *SNF-TEST-CODE-60-H4lxEBwzaP2Qmz* 1063379 *SNF-TEST-CODE-59-KOGZdIbcgH7CGW* 1063378 *SNF-TEST-CODE-58-70TAjQNGIvoVOQ* 1063376 *SNF-TEST-CODE-57-9mLZaRQzYXK0It* 1063375 *SNF-TEST-CODE-56-WC1pEpJ9blQWPV* 1063372 *SNF-TEST-CODE-55-OhjHXyXfKMyLAv* 1063371 *SNF-TEST-CODE-54-Yy4OAGMWhZpY8b* 1063370 *SNF-TEST-CODE-53-oLWCj7U9npJVNR* 1063368 *SNF-TEST-CODE-52-EBfFdU6OwLyHvr* 1063366 *SNF-TEST-CODE-51-pPhvWVmdRQH9WN* 1063364 *SNF-TEST-CODE-50-szoRd6CDLAWr4T* 1063362 *SNF-TEST-CODE-49-OPYzwVsSCyoEjR* 1063356 *SNF-TEST-CODE-48-EEmCj2XpVtVZUE* 1063353 *SNF-TEST-CODE-47-t18wBPpwNgJb61* 1063349 *SNF-TEST-CODE-40-GdnCFvZMnezIcO* 1063346 *SNF-TEST-CODE-20-l04E4AU6064xjz*