Found the solution myself:

It was a browser issue all along. Karma/Jasmine are behaving just fine, and open links in a new window/tab as my tests intend.

Instead, the browser's popup blocker was at fault. I had thought that I had ruled out popup blockers before posting here, because I did not see any notifications about blocked windows. Turns out that Chromium (which I used for testing) does not show such notifications, but Firefox does. At least in my respective configurations.

Setting up exception rules has resolved the issue for both browsers.

My bad.