Nevermind, I figured it out.  There's a lovely bit of code in   scala/collection/parallel/package.scala:

{code}
private[parallel] def getTaskSupport: TaskSupport =
  if (scala.util.Properties.isJavaAtLeast("1.6")) {
    val vendor = scala.util.Properties.javaVmVendor
    if ((vendor contains "Oracle") || (vendor contains "Sun") || (vendor contains "Apple")) new ForkJoinTaskSupport
    else new ThreadPoolTaskSupport
  } else new ThreadPoolTaskSupport
{code}

If I change that to just
{code}
       private[parallel] def getTaskSupport: TaskSupport = new ForkJoinTaskSupport
{code}

the parallel-collections test passes on Avian and HotSpot. However, if I change it to

{code}
       private[parallel] def getTaskSupport: TaskSupport = new ThreadPoolTaskSupport
{code}

it hangs on both Avian and HotSpot.  In other words, the test simply won't complete when using ThreadPoolTaskSupport, regardless of the VM, which means the test can only pass if the VM vendor is Oracle, Sun, or Apple.

Ouch.