wasi: implement readdir

[codefromthecrypt]

Until readdir is implemented in wasm, we can't re-use go's utilities for testing for bugs in impl.

2022/01/01 00:00:00 TestFS err: TestFS found errors:
        .: ReadDir(-1): readdir unimplemented : errno 54

Here's what would be nice to be able to both compile and run (right now, it compiles but can't run).

package main

import (
	"fmt"
	"log"
	"os"
	"syscall"
	"testing/fstest"
)

func main() {
	if err := fstest.TestFS(os.DirFS("sub"), "test.txt"); err != nil {
		log.Panicln("TestFS err:", err)
	}
	fmt.Println("TestFS ok")
}

Maybe implementing needs a change here https://github.com/tinygo-org/tinygo/blob/d984b55311a2acd6c1b14ef6e87ee280e737a925/src/syscall/syscall_libc_wasi.go#L263-L265