selftests: Add header documentation and helpers

Add "how to use this API" documentation to kselftest.h, and include some
addition helpers and notes to make things easier to use.

Additionally removes the incorrect "Bail out!" line from the standard exit
path. The TAP13 specification says that "Bail out!"  should be used when
giving up before all tests have been run. For a "normal" execution run,
the selftests should not report "Bail out!".

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