[Thread] refactor IntegrationTestUtils to use Kotlin

This CL refactors IntegrationTestUtils class to Kotlin. As more test
cases are introduced, it's likely we'll add more code into this class.
It's worth a refactor of the code so that we can write the new code in
Kotlin which is more concise and safer.

Change-Id: I8b71c5183cad9224ee49e2192aa5aab62aae9ddb
3 files changed