Open browsers in the background

It would be oh so nice if redacted:project_name opened browsers in the background instead of the foreground, so we don't have to move focus back to the IDE/terminal

On mac at the very least, you can open browsers in the background: `open -g -a "Google Chrome"` as an example. I'll assume this is possible on windows/*nix also.

The issue does not name a file, test, or entry point. Start by locating Karma's browser-launching logic and determine how background opening can work on macOS, Windows, and Unix-like systems; done means launched browsers no longer take focus where the platform supports it, with coverage for the supported behavior.

Written by the indexing model from the issue text.