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 <tjmercier@google.com>
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Author: Muhammad Usama Anjum