tests: Simplify 4k tests setup and teardown
    
    Previously you had to setup the environment manually to enable the 4k
    tests, and pass environment variable to tests. The tests used single
    path, so you had to run the tests twice to test both 4k block device and
    filesystem backed by a 4k block device.
    
    Add storage helper script to make it easier to setup storage for the
    tests.
    
    To setup 4k storage for the tests, run:
    
       $ python tests/storage.py setup
    
    This script creates both a 4k block device and filesystem backed by a 4k
    device. The user_4k_path() fixture enables the tests if the device and
    mount exist, and runs the test twice, once with the loop device, and
    once with the filesystem.
    
    To clean up the environment, run:
    
       $ python tests/storage.py teardown
    
    Signed-off-by: Nir Soffer <nsoffer@redhat.com>