Add infrastructure for parallel testing

For parallel UMB testing, we need kpet to be in a separate stage to retrieve a list of modified files to pass along in the message (the message will be sent in additional "test" stage).

Separate out kpet into extra stage that runs before testing, add placeholder anchors for modified file retrieval and UMB messaging, and adjust all tree YAMLs to the new definition.

Signed-off-by: Veronika Kabatova vkabatov@redhat.com