See https://groups.google.com/a/chromium.org/d/topic/chromium-dev/vDde4zO24eg/discussion for context.

At one point, you could actually run a try job and tell it that you only wanted to run a single test, but we stopped supporting that.

We should make it possible to do that again. The thread referenced above tells how you can do it without needing any infra changes, but we can clearly make it a lot easier to handle either by scripting things better, or by making changes to the recipes (e.g., to set properties on the build).

You can also do this by triggering jobs on swarming directly, but that requires you to be able to build locally, which only works in some situations.