In build flow jobs, a post-build step to aggregate build flow test results (of jobs invoked by the build flow job) is available. Can a pipeline job aggregate test results of jobs invoked by the "build" step as well?
It could, though generally if you are asking for this you should rather be considering doing the aggregated work in the same Pipeline job, rather than using the build step to chain together separate jobs.
+1 - I have two separate pipelines that are triggered with a restricted frequency i.e. one is run daily, the other weekly (and eventually this will also be expanded to be monthly and yearly) and I want to be able to aggregate the test results based upon a common artifact in these tests that have been run with different frequencies. I don't believe that this use case is possible with pipeline?
It is possible, if a bit awkward—you would need to archive the JUnit result XML files, use Copy Artifact to copy them into into the workspace of another job, and then run the junit step on each result set. (Multiple calls to junit from a single Pipeline build are aggregated automatically. There is a separate RFE open to allow them to be visually distinguished.)
Considering that pipeline is now THE way to go with Jenkins, the feature would greatly simplify the flow for people implementing pipeline for specific usage.