When you are in a social / public situation and have the urge to pass a considerable amount of gas. If it has been a long time since your last fart, you are unsure of:
1) The sound level (volume) of the fart
2) The smell level (stench) of the fart
... so you let out a small amount of the fart in a discreet fashion- thus you have let a test fart.
Frank was in church the other day and had to fart really bad. He said he intended to let a test fart but instead of a squibbler, it turned out to be a blastus and he wound up shitting his pants in the middle of the sermon. Damn, what an idiot!
Basically when you're around people (not alone) and you really have to blast one, so much so that your stomache is starting to ache... so you let out a small amount of gas in hopes of it being a quiet or non-smelly fart. If it smells or is a blastus you hold it back, but if it's a quiet one you walk around and pass the gas in a discreet fashion.
Hung was in class earlier and he had to fart, he intended to let out a test fart in the back of the class but instead of is coming out as a quiet and/or non-smelly fart he accidently Sharted.