Explicitly select the "du" block size in t/test.sh.

Since du's default output can be affected by several environment
variables (DU_BLOCK_SIZE, BLOCK_SIZE and BLOCKSIZE), be explicit about
what we want via -B 1024.

Thanks to Michael Ekstrand <michael@elehack.net> for the original
report, and help with the fix.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>