I started a few runs of the test locally, though didn't do a full run. From these quick experiments it looks as if the bg index build goes too fast for the checks in the test to complete.

<http://buildbot.mongodb.org/builders/Linux%2064-bit%20v8%20Weekly%20Slow%20Tests/builds/79/steps/test/logs/stdio>
<http://buildbot.mongodb.org/builders/Linux%2064-bit%20v8%20Weekly%20Slow%20Tests/builds/78/steps/test/logs/stdio>
<http://buildbot.mongodb.org/builders/Linux%2064-bit%20v8%20Weekly%20Slow%20Tests/builds/77/steps/test/logs/stdio>

Test seems to pass ok on the non v8 weekly buildbot. The behavior difference could potentially be an issue of machine difference not javascript engine difference.