selftests/harness: Switch to TAP output

Using the kselftest_harness.h would result in non-TAP test reporting,
which didn't make much sense given that all the requirements for using
the low-level API were met. Switch to using ksft_*() helpers while
retaining as much of a human-readability as possible.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>