remove unused code in AgentSet classes

not just unused, but could be dangerous if used -- see comment removed
from ArrayAgentSet
fix theoretical bug in new StructureParser

might affect error messages under some circumstances, not really sure
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 commit '189d12c3173bbf61a631461ff01b30cf2ccfc531' into headless

Conflicts:
	headless/src/main/org/nlogo/agent/World3D.java
Merge commit '815b3d94d4f19c02b191a8a781aa6bc73b844309' into headless

Conflicts:
	headless/src/main/org/nlogo/parse/StructureParser.scala
Merge commit 'ea848a32bdf944d341aadc50105eef2809ff4e7b' into headless