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/abort.wren

@@ -0,0 +1,7 @@
+var fiber = new Fiber {
+  Fiber.abort("Error message.")
+}
+
+IO.print(fiber.try()) // expect: Error message.
+IO.print(fiber.isDone) // expect: true
+IO.print(fiber.error) // expect: Error message.