tests: try using truncate command before falling back to using dd

tests: try using truncate command before falling back to using dd

On the GNU Hurd, dd seems to be buggy and hangs if seeking beyond 4G.
Fortunately the GNU Hurd does have the truncate command, which does
work correctly.  So try using the truncate command first, and fall
back to using dd only if the truncate command doesn't work.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>