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/resume_caller.wren

@@ -0,0 +1,19 @@
+var b = new Fiber {
+  IO.print("fiber b")
+}
+
+var a = new Fiber {
+  IO.print("begin fiber a")
+  b.call()
+  IO.print("end fiber a")
+}
+
+IO.print("begin main")
+a.call()
+IO.print("end main")
+
+// expect: begin main
+// expect: begin fiber a
+// expect: fiber b
+// expect: end fiber a
+// expect: end main