I've run into a couple problems in IntelliJ 12.x when working on Scala projects with ScalaTest. The test runner normally works wonderfully, but rarely I'll get into a state where, when attempting to run a test configuration for a Suite I get an error message:

One or more requested classes are not Suites

This occurs for test suites that previously were executing just fine in IntelliJ and the suites continue to execute within an sbt session.

To fix this problem I quit IntelliJ and re-run sbt gen-idea. Then restart Intellij and after completing its background tasks the test suites will be runnable with the test runner once again.

(When I can isolate what puts IntelliJ into this state I'll file a bug report.)