If you've seen this error after trying to run individual Android implementation tests or test classes (by clicking on the run icon in the gutter) in a Kotlin project in Android Studio 3.0.1, you can get around the error by instead running the full test package (by right-clicking on the test package and choosing "Run tests in...").
There is a known bug in Android Studio 3.0.1 that causes the IDE to run Kotlin implementation tests as regular JUnit tests, which caused the OP's error message to get shown on my machine. The bug is tracked at https://issuetracker.google.com/issues/71056937. It appears to have been fixed in Android Studio 3.1 Canary 7.