Fix directory creation race condition

[rickclephas]

Test generation has been parallelized in https://github.com/JetBrains/kotlin/commit/bb8a46c3a0d69ffe57abbbe43e28afd434430fdf. This however creates a race condition in case a directory needs to be created for multiple tests. mkdirs only returns true if the directory was actually created, adding an additional check on the existence of the directory should solve that.