Starting in kotlintest 3.0, the library was split into separate modules so that you can include only the parts that you need, and to support more configurations.
https://github.com/kotlintest/kotlintest/blob/master/CHANGELOG.md#version-30x---march-29-2018
If you want to use kotlintest assertions without the runner, you can use
testCompile 'io.kotlintest:kotlintest-assertions:3.1.10'
If you want to use the test runner, then you should depend on
testCompile 'io.kotlintest:kotlintest-runner-junit5:3.1.10'
. That dependency brings in the core and assertions modules transitively.