You were unreasonable and I think you know that - I agree your DS should have stayed in the year 6 classroom to do his test but you should have made sure of that. The school will then just have to catch up his 1:1 another time.
How on earth could OP make sure of that?
BUT to all the posters complaining about the TA being used for cover, this is almost certainly their only option. They can't take a TA who is fulltime 1:1 with a child with medical or severe behavioural needs because it isn't safe for them not to have that 1:1. The OPs DS just gets 6 hours a week so they legally can, as long as they make it up.
No, it isn't their only option. It isn't an option at all, because it is unlawful.
You're assuming this TA has no other responsibilities. I think that's highly unlikely - it may well be that supports one or more other children when not with OP's son, so even ore children are potentially being severely short-changed.
Lots of pps saying well run schools have cover arrangements. Not any more they don't. There are NO supply teachers. It doesn't matter how well run the school is, or whether they give half an hour's notice or 2 days - there just aren't the staff anymore, supply agencies are telling us they have no one. They can't be magicked. This is the reality of education now.
Then other arrangements should have been made, as pointed out above - e.g. having the head or deputy covering the Y3 class or this child's 1:1, asking a part time teacher to work extra, putting two classes together etc. School have been misusing 1:1 support for SEN children for decades, this is not something new, and they need to get out of the mindset that the law doesn't matter, nor does failing children with difficulties.