docs/kselftest: add more guidelines for adding new tests

Improve and add instructions to add new tests. Add build commands to
test before sending the new test patch.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20220521073651.4191910-1-usama.anjum@collabora.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>