There are two subtly different things you might want to test.
- Is Snort working in the sense that it's running, able to sniff trafic, testing it against the rules, and alerting you when one is triggered?
- Is Snort working in the sense that it's current rule set detects a specific intrusion of type X?
To test case 1, you make a rule that's easy to fire, like your example, and fire it.
To test case 2, you have to attempt an intrusion of type X and confirm that it is detected.
You seem to be wanting to test case 1 (that the install has been done correctly) using the method in case 2, but you don't need to. Using a "fake" rule is a perfectly valid test that Snort is working in the first sense. And it's easier. Easy tests are good. You don't want to faff around with Metasploit when you're just checking that the alert emails go to the right person. Especially if you're not skilled in running intrusions - what if you do the intrusion wrong, and get a false test result? What if the intrusion attempt crashes the target (which is very likely on many types of intrusion.)
You really only need to test case 2, that a specific rule works against a real intrusion attempt, if you don't trust your rule set (in which case - why are you using it?) or if you're developing new rules.