kunit: tool: Add (primitive) support for outputting JUnit XML

kunit: tool: Add (primitive) support for outputting JUnit XML

This is used by things like Jenkins and other CI systems, which can
pretty-print the test output and potentially provide test-level comparisons
between runs.

The implementation here is pretty basic: it only provides the raw results,
split into tests and test suites, and doesn't provide any overall metadata.
However, CI systems like Jenkins can ingest it and it is already useful.

Link: https://lore.kernel.org/r/20260606013827.240790-2-david@davidgow.net
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: David Gow <david@davidgow.net>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>