Trying to make AsynchPeopleTest more robust.

https://jenkins.ci.cloudbees.com/job/core/job/jenkins_main_trunk/1369/testReport/junit/hudson.model/AsynchPeopleTest/display/
org.junit.ComparisonFailure: expected:<[display: none;]> but was:<[]>
	at org.junit.Assert.assertEquals(Assert.java:115)
	at org.junit.Assert.assertEquals(Assert.java:144)
	at hudson.model.AsynchPeopleTest.display(AsynchPeopleTest.java:44)
was presumably a race condition.
Could reproduce by adding a Thread.sleep into AsynchPeople.compute.
This patch (with waitForBackgroundJavaScript) avoids that error;
but causes the assertNotNull at the end to fail, for reasons TBD.