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/language/fiber/closure.wren

@@ -0,0 +1,25 @@
+var fiber
+var closure
+
+{
+  var a = "before"
+  fiber = new Fiber {
+    IO.print(a)
+    Fiber.yield()
+    a = "after"
+    Fiber.yield()
+    IO.print(a)
+    a = "final"
+  }
+
+  closure = new Fn {
+    IO.print(a)
+  }
+}
+
+fiber.call()   // expect: before
+closure.call() // expect: before
+fiber.call()
+closure.call() // expect: after
+fiber.call()   // expect: after
+closure.call() // expect: final