test/test-functions: use truncate instead of dd to create testbed image

test/test-functions: use truncate instead of dd to create testbed image

This avoids unnecessary noise in the stderr logs which dd always produces,
such as:

0+0 records in
0+0 records out
0 bytes copied, 0.000155284 s, 0.0 kB/s

Using truncate should not result in any functional change; the image will
still be created as a sparse file of the size specified.