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
Merge pull request #428 from NetLogo/scalatest-2.0.M6-upgrade

upgrade master to ScalaTest 2.0.M6