We should have a patronus test in 3rd year

Patronuses aren't taught in the third year yet. On EU, the patronus lesson only seems to be in the seventh year. Just look at the description: "It is an immensely complicated and extremely difficult spell that evokes a partially-tangible positive energy force known as a Patronus," I don't think third years can cast this yet. 

Also, some people may want to choose their own patronus. Tests aren't always accurate and they may get a result they're very unhappy with.

i think that is more up to the teacher but what eve said. further more i think that it would be more of guidelines then actual rules. like you get a link to an offsite test in the lesson