selftest: Show expected and actual return codes for test failures in fcnal-test

Capture expected and actual return codes for a test that fails in
the fcnal-test suite.

Signed-off-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240124214117.24687-4-dsahern@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>