Xcode 10 fails my tests in simulator without any information as to the failure

This answer seems to have worked much like the poster saw the deployment targets seemed off for the test targets.

The test seem to run fine in an XR simulator, which should be iOS12.

Clean Build Folder helps to me: In Xcode go to Product -> Clean Build Folder

Deleting the app, restarting both Xcode and device worked for me.

I had the same problem than you and I've found a solution.

In Xcode go to:

Xcode > File > Workspace Settings...

or

Xcode > File > Project Settings...

Change Build System (in shared workspace settings and in per-user workspace settings) from New Build System (default) to Legacy Build System.

I hope this will help somebody in the future :)