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.

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.