selftests/dmabuf-heap: conform test to TAP format output

Conform the layout, informational and status messages to TAP. No
functional change is intended other than the layout of output messages.
Improve the TAP messages as well.

Reviewed-by: T.J. Mercier <[email protected]>
Signed-off-by: Muhammad Usama Anjum <[email protected]>
Signed-off-by: Shuah Khan <[email protected]>

Author: musamaanjum