Bugfix add timeout to core.FreestyleJobTest#discardBuilds

[jmdesprez]

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FreestyleJobTest#discardBuilds is flaky because of org.jenkinsci.test.acceptance.po.Build#waitUntilFinished() is not waiting for the build to be actually finished but completed. This is related to https://github.com/jenkinsci/jenkins/pull/6829 and JENKINS-68981 as there is currently no (easy/reliable) way to wait for a build to be finished.

The problem is that org.jenkinsci.test.acceptance.po.Build#waitUntilFinished() exits before logRotate -> LogRotator#perform gets called, leading to a race condition

If assertThat(b2, pageObjectDoesNotExist()); is called before LogRotator#perform then the test fails with this stacktrace

java.lang.AssertionError: Expected: Page object exists
     but: http://mvn:64648/job/bald_ski/2/ does exist
at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:20)
at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:8)
at core.FreestyleJobTest.discardBuilds(FreestyleJobTest.java:200)

Running the ATH with INTERACTIVE=true, shows that the build is removed as expected.

The workaround add a timeout to mitigate the race condition.