test: Avoid using 'truncate' command

test: Avoid using 'truncate' command

The 'truncate' command is not available on all platforms. Since the
command was being used to zero out a file, there are other easy options
available. In this case, I've replaced 'truncate' with a redirection.

Reported-by: Alin Gabriel Serdean <aserdean@ovn.org>
Signed-off-by: Mark Michelson <mmichels@redhat.com>
Tested-by: Alin Gabriel Serdean <aserdean@ovn.org>
Acked-by: Alin Gabriel Serdean <aserdean@ovn.org>
Signed-off-by: Alin Gabriel Serdean <aserdean@ovn.org>