Documentation: dev-tools: Add KTAP specification

Documentation: dev-tools: Add KTAP specification

It does not make any significant additions or changes other than those
already in use in the kernel: additional features can be added as they
become necessary and used.

[1]: https://testanything.org/tap-version-13-specification.html

Signed-off-by: Rae Moar <rmoar@google.com>
Co-developed-by: David Gow <davidgow@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20211207190251.18426-1-davidgow@google.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>