Skip testDirectory on S3

This test cannot succeed due to the way directories are implemented as
prefix pseudo-files in S3.
Allocate new container name using prefix

Before we used the previous container name, which kept appending and
hit S3 limits during integration tests like:

Caused by: java.lang.IllegalArgumentException: Object 'gaul-blobstore27-4348532805015069992-1155797412711190448-1465016400636204031' doesn't match S3 bucket virtual host naming convention. Reason: Can't be null or empty. Length must be 3 to 63 symbols.. For more info, please refer to http://docs.amazonwebservices.com/AmazonS3/latest/index.html?BucketRestrictions.html