Need junit.xml in both submission.tar.xz and the one produced by checkbox-cli

[Alex Tu]

the jenkins is counting on junit.xml to generate summery.html, and junit.xml will always be produced after testflinger executing checkbox testing.

But when user need to merge several results then there's a problem that no merged junit.xml there.

For example while user want to test com.canonical.certification::sru then com.canonical.certification::graphics-discrete-gpu-cert-automated and merge their result by

$ checkbox-cli merge-submissions ./submission.tar.xz ./submission_dgpu.tar.xz -o ./submission.tar.xz --title "Full"

User will not get a junit.xml which merged from those 2 plan so user can not get a summery.html by job-summary

ex. https://taipei-jenkins-docker.redirectme.net/view/Daily-Auto-Sanity/job/sanity-3-testflinger-dell-bto-bionic-bionic-master-201809-26471/