ext: Add failure node to JUnit xml file

ext: Add failure node to JUnit xml file

"failure" is a child of the testcase node:

https://llg.cubic.org/docs/junit/

It allows xml parsers to understand which testcase failed the run.
Otherwise CI frameworks like jenkins wouldn't be able to classify every
single testcase. Prior to this patch testlib was using
testsuites.failures and testsuite.failures only. These are simply
reporting the number of failures.

Change-Id: I0d498eca029c3232f2a588b153b6b6829b789394
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25083
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>