Align test framework with that of detekt

[Whathecode]

Originally, I used Spek since this is the test framework detekt used. In case they would ever want to incorporate these rules in the base library.

But, they have now migrated away from Spek. This library should thus probably do the same.

While doing so, the createKotlinCoreEnvironment() calls in tests should also be replaced with @KotlinCoreEnvironmentTest on test classes. I tried doing this with Spek, but the tests would hang indefinitely.