Documentation/dev-tools: Add kselftest_harness documentation

Add ReST metadata to kselftest_harness.h to be able to include the
comments in the Sphinx documentation.

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Will Drewry <wad@chromium.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>