Build Flow Test Aggregator is the simplest way to aggregate test results from builds started dynamically by the build flow job. It's a post-build step that only shows up for build flow projects.
Normally to aggregate test results Jenkins has to "understand" the upstream - downstream dependencies between jobs. This can be achieved by generating an artifact in job1 and fingerprinting that archive from all the downstream jobs. But even then you'd have to have a separate job where you aggregate test results, which is not convenient.
Build Flow Test Aggregator simply loops over all of the builds scheduled by a flow run and retrieves and aggregates test results if there are any. The test results will be available just like any other test results, it will show aggregated results and statistics, with links to each individual test for further inspection:
To configure test aggregation you need to set the "Flow run needs a workspace" checkbox:
Then just enable the "Aggregate build flow test results" post-build action: