Tweak a few things in the new GC.

- Make sure it handles an empty gray set.
- Make sure growing the gray stack doesn't itself trigger a GC.
- Make sure it works when stress testing is enabled.
- Ensure the tests kick off a GC.

test/language/deeply_nested_gc.wren

@@ -4,4 +4,5 @@ for (i in 1..400000) {
   head = { "next" : head }
 }
 
-System.print("done") // expect: done
+System.gc()
+System.print("done") // expect: done

test/language/many_reallocations.wren

@@ -10,5 +10,7 @@ for (i in 1..1000) {
   }
   found.add(bar)
 }
+
+System.gc()
 System.print(found.all {|i| i == 1337}) // expect: true
-System.print("DONE!") // expect: DONE!
+System.print("done") // expect: done