test fart
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!