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/benchmark/fib.wren

@@ -0,0 +1,12 @@
+class Fib {
+  static get(n) {
+    if (n < 2) return n
+    return get(n - 1) + get(n - 2)
+  }
+}
+
+var start = IO.clock
+for (i in 1..5) {
+  IO.print(Fib.get(28))
+}
+IO.print("elapsed: ", IO.clock - start)