Replace 'dd' in tests with use of guestfish 'sparse' command.

'dd' is a very inefficient way to create files.  'truncate' is
better, but unfortunately that command is not available in RHEL 5.
So use the guestfish 'sparse' command instead (which also avoids
allocating disk space).
(cherry picked from commit 175c01d2336c07113f1c2966966957de5c344e3a)