help stuff in PlotManager get GC'ed faster

I don't think this is strictly necessary since the references are
weak, but it does mean less false leads to track down in VisualVM.
(though I've since switched to YourKit which can automatically elide
weak references in heap dumps)
make TestHalt more robust
add some new, simpler tests to TestHalt

if the most complex one fails, it's helpful to know if the simpler
forms fail too or not