Add proper tests for compile-time errors in `sample-kotlin-usage`

[tlinkowski]

Currently, we can see the compile-time errors in sample-kotlin-usage in IntelliJ only (in Gradle, compileKotlin was disabled for the build to work). In the future, we'd like to make a full-blown test set out of this project (so that the tests verify that the compile-time errors are where they're expected).