Application.isPlaying in Test Runner - Check if running Test Runner

I've been using Application.isPlaying in some of my scripts to check whether the game is running or unit tests are. (This allows me to execute alternative behaviours and return dummy values for Unity API stuff, like FindObjectsOfType.)

However, while batch testing, the Test Runner might move away from the main thread, making Application.isPlaying calls illegal. This leaves me with tests that succeed if run on their own, but that might fail if run in a batch.

What's the simplest way to check whether I'm running a unit test ?