kunit: tool: Add test for nested test result reporting

kunit: tool: Add test for nested test result reporting

Currently there is a lack of tests validating the result reporting from
nested tests. Add one, it will also be used to validate upcoming changes
to the nested test parsing.

Link: https://lore.kernel.org/r/20251230-kunit-nested-failure-v1-1-98cfbeb87823@linutronix.de
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>