test/py: correctly log xfail/xpass tests

Tests can complete in passed, skipped, xpass, xfailed, or failed, states.
Currently the U-Boot log generation code doesn't handle the xfailed or
xpass states since they aren't used. Add support for the remaining states.
Without this, tests that xfail end up being reported as skipped.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>