upgrade to ScalaTest 2.0.M6

I forget why I got the master branch on the ScalaTest 2.0 milestone/RC
upgrade train in the first place, but nonetheless, now that we're on
it, I feel responsible that we should stay on it until we have arrived
at a stable 2.0 release

(some good news is, I've been using 2.0 milestones and snapshots on
the headless branch witp no real problems)

changes needed to get compiling/running again:
- FunSuite is now a class not a trait, so I had to adjust the
  "extends" clause of some of our test stuff, and make a few of our
  classes into traits
- due to deprecation, renamed expectResult -> assertResult everywhere
- fixed a mistaken return type for a use of withFixture
review tab: re-enable save button on FrameAddedEvents
review tab: move refresh buttons code in separate method
review tab: explicitely make _runs a Vector
review tab: safer diaglogs  when closing runs - fix #373
review tab: re-organize imports

Going back to one import per line. It takes up more space, but it makes for
nicer diffs in git, where it's easier to see what's been added removed.
review tab: thread more carefully in RunRecorder
review tab: make sure all new runs are caught - fix #372