Documentation: kunit: Add naming guidelines

As discussed in [1], KUnit tests have hitherto not had a particularly
consistent naming scheme. This adds documentation outlining how tests
and test suites should be named, including how those names should be
used in Kconfig entries and filenames.

[1]:
https://lore.kernel.org/linux-kselftest/202006141005.BA19A9D3@keescook/t/#u



Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Reviewed-by: Tim Bird <tim.bird@sony.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>