The 5V wall wart Chuck mentioned is what I had in mind. It has a transformer that electrically isolates the secondary (5VDC source) from the other transformer powering the switch machine, and in your case the test dongle. If you connect the wall wart's output one way you get 5VDC and if you swap the connections, it becomes -5VDC. Because it's electrically isolated, it doesn't care if one of it's legs is tied to the track power common (black wire on turnout), which is what needs to be done.
The other point you make about the switching for the turnout, yes that switch is a momentary SPDT. Electrically, what it does is make a brief short between the black ground and either the (Red - Out or the Green - Through) turnout's terminals to activate a throw of the turnout's position.
