atf_pytest: fix xfail detection from pytest report

The location of the 'wasxfail' attribute was moved from
the 'reason' attribute back to the parent 'report'. This
fixes an issue where xfails are wrongly reported to ATF
as skipped tests.

Signed-off-by: Siva Mahadevan <me@svmhdvn.name>
Pull-request: https://github.com/freebsd/freebsd-src/pull/1849
Sponsored by: The FreeBSD Foundation
(cherry picked from commit df997faca8411bdf8008c25de5db3ed719019d21)