Allow configuring the target name for a sample directory

[wolfs]

After #6 is merged, there is still one use-case missing for replacing the Sample Rule in gradle/gradle: The Sample rule allows configuring testSampleDirName, the target directory name for the sample.

I tried removing that functionality in the gradle/gradle code base, but I ran into long path issues on Windows: https://github.com/gradle/gradle/pull/5713.

So there should be a possibility to configure the target directory where a sample is copied to, either by specifying a fixed relative path to the target base dir or a target directory directly.

I think using a target directory supplied as a File would suffice. This field could then be wired in the method computeSampleDir.