Canada

Our section 11c allows for the defendant's right not to give testimony directly against oneself,

11 Any person charged with an offence has the right

(c) not to be compelled to be a witness in proceedings against that person in respect of the offence;

but it doesn't allow for one defendant not to give testimony against another -- just, via section 13, indemnification against that witness in a separate proceeding when previous testimony is compelled (via lack of a right to silence when potentially incriminating others).

13 A witness who testifies in any proceedings has the right not to have any incriminating evidence so given used to incriminate that witness in any other proceedings, except in a prosecution for perjury or for the giving of contradictory evidence.

So, cops don't have to testify against themselves, but they don't get to be silent. And they get indemnification if they do speak up, so it seems a dumb move not to say "yeah, me and Bobby totally beat the hell outta that guy" and use that section-13 clause to skate.

Hmm. But they'd be bounced out of the force on Ethics, but I guess at that point they're no longer cops anyway; just thugs (and if you're all ACAB about it, you need to know why that's toxic).