cmd/core/dd: fix dd_test to write data file to tmp
cmd/core/dd: fix dd_test to write data file to tmp

writing test file to "current" directory can lead
to indeterministic test run results in places.

Signed-off-by: David Hu <a59597aa737a9c4d32eaf7b77687719fab10c4c5@google.com>

u-root/u-root,u-root/u-root,u-root/u-root