compileSourceRoots in testCompile should be writable

In projects with Java and Kotlin sources, it should be possible to set the `compileSourceRoots` on the plugin level and not the project level. This way the Kotlin compiler can be configured to only consider Kotlin sources and the Java compiler can only consider Java sources.

`compileSourceRoots` was already changed to writable in the `compile` goal. So I don't see any risks in also changing it for the `testCompile` goal.