selftests/mm: mlock2-tests: 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. 
I've done some cleanups as well.

Link: https://lkml.kernel.org/r/20240202113119.2047740-6-usama.anjum@collabora.com


Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>