Reorganize tests and benchmark scripts.

Mainly to get rid of one top level directory. But this will
also be useful when there are tests of the embedding API.

test/core/fiber/run.wren

@@ -0,0 +1,9 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+}
+
+IO.print("before") // expect: before
+fiber.run()        // expect: fiber
+
+// This does not get run since we exit when the run fiber completes.
+IO.print("nope")