Allow merging all stages result xml

[eshulman2]

Polarion shows stages results as different test run which makes it
harder to manage. Allow merging all stages results if required

Solves: https://issues.redhat.com/browse/NFV-3317

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[tosky]

Just for the record, I think the real solution to this would be the implementation of "test phases", which would allow us to aggregate test results independently from how test operator handles the tests.
The idea would be to define test results as belonging to a certain test phase and aggregate based on that.

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https://softwarefactory-project.io/zuul/t/rdoproject.org/buildset/a34d2fc7544a4331afe239963aac9d15

✔️ openstack-k8s-operators-content-provider SUCCESS in 1h 40m 27s
✔️ podified-multinode-edpm-deployment-crc SUCCESS in 1h 11m 43s
✔️ cifmw-crc-podified-edpm-baremetal SUCCESS in 1h 27m 10s
✔️ noop SUCCESS in 0s
✔️ cifmw-pod-ansible-test SUCCESS in 9m 14s
❌ cifmw-pod-pre-commit FAILURE in 8m 26s
✔️ build-push-container-cifmw-client SUCCESS in 16m 37s
✔️ cifmw-molecule-polarion SUCCESS in 2m 19s