WaitForCompletion support for spawned test units

When spawning multiple test units, it would be nice to have a simple mechanism to wait for them to complete.
Currently, we only have the option to wait for all the threads in the VM to stop (ParallelUnit.waitForParallelZero()),
or the more complicated option of WaitForBarrier (not everyone understands how that works).

Sample code:

WaitForCompletion with
 %Timeout% 60000
 %Units% with
  %DoSomething% as spawn unit.web.SomeUnit
  %DoSomethingElse% as spawn unit.web.SomeOtherUnit
 end
end

When the timeout comes, the following could be displayed:
Error: Unit "DoSomething" did not complete