Its illegal to not tell everything you know in court. If you say nothing, you might get sued. And if you say you don't know anything, you're fucked when another witness says you knew it.

There are only three exceptions:

You might get yourself in trouble, in that case you don't have to answer the question
It's a case against a family member in which case you also don't have to testify.
It would go against your professional secrecy

But you have to testify against a friend.

I just looked it up and it's exactly the same in the US. So you also can be force to testify in Murica