[JENKINS-53172] Make sure the reference build is not running

[raul-arabaolaza]

JENKINS-53172

The code to choose a previous run as reference is as follows:

private static TestResult findPreviousTestResult(Run<?, ?> b, TaskListener listener) {
        for (int i = 0; i < NUMBER_OF_BUILDS_TO_SEARCH; i++) {// limit the search to a small number to avoid loading too much
            b = b.getPreviousBuild();
            if (b == null) break;
            if(!RESULTS_OF_BUILDS_TO_CONSIDER.contains(b.getResult())) continue;

            AbstractTestResultAction tra = b.getAction(AbstractTestResultAction.class);
            if (tra == null) continue;

            Object o = tra.getResult();
            if (o instanceof TestResult) {
                listener.getLogger().printf("Using build #%d as reference%n", b.getNumber());
                return (TestResult) o;
            }
        }
        return null;    // couldn't find it
    }

Which AFAIUI does not perform any check about the running status of the potential reference run

As per javadoc #getResult alone is not enough guarantee that the run is finished as it can return a value even if the job has not yet finished. And I have found situations where a previous, still running run, is used as the reference. With the consequence of improper parallelization because not all tests have finished (only some of the branches).

This PR updates the aforementioned code to add a check for discarding running jobs as reference, created with the intention of get some feedback as I am still running tests on my instance.

[raul-arabaolaza]

All my tests indicate that the problem is not happening again.

[raul-arabaolaza]

Pinging @abayer and @jglick as you are the last commiters I can see in the repo

[abayer]

Any chance you could add a test too? Thanks!

[raul-arabaolaza]

@abayer New test for you to review :)

[abayer]

Looks good to me.