Documentation: kunit: update kconfig options needed for UML coverage

Recent changes have made it so the current set is not sufficient.
Namely, CONFIG_DEBUG_INFO is not being set even when explicitly asked.

Specifying a version of the debug info fixes this.
Pick CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT as an option that's
hopefully less fragile (esp. given we're tied to GCC 6 and lower).

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Tested-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>