A goldfish is a rather boring pet: it doesn't do much, and you can't really interact with it. It just swims around in its bowl, and then one day the cat eats it or you come in and find it floating belly-up.
The most basic deck-test (originally called "The Test" on Usenet) is very much like a goldfish: it's an opponent who doesn't do anything. All you have to do is deal 20 points of damage as quickly as possible. (If your deck can't deal 20 points of damage against a defenseless opponent, it's time to give up Magic and start playing Go Fish. Unless, of course, it's a "jujitsu" decksee below.)
Count how many turns it takes to kill the fish. Do this several times, and average the results to get a measure of the sheer brute power of your deck. Five or less is an amazing score, usually possible only with extremely lucky shuffles or a deck loaded with out-of-print cards. A more typical fast deck will score seven or eight fairly consistently. If your deck consistently scores over ten, you're likely to get chewed up by faster decks.
Exceptions to this are jujitsu-style decksthose that do almost nothing on their own, but turn all your opponent's forces against him. None of these tests will really give a good measure of a jujitsu deck's abilities. Also, the Goldfish opponent has no hand, so Black Vise and The Rack don't deal any damage and Hymn to Tourach is pointless. Decks based around these concepts will do better against real opponents than the Goldfish test would indicate. But even a jujitsu or hand-manipulation deck should have a few cards capable of dealing normal damage in case of an uncooperative opponent, so the test is still fair.
