How to wait until a specific line is printed to os.Stderr?

I'm running a Goroutine which, after some delay, logs a specific line to os.Stderr. I'd like to wait until that line is logged. So far, what I've tried is

package main

import (
	"bufio"
	"log"
	"os"
	"strings"
	"time"
)

func main() {
	go func() {
		time.Sleep(time.Second)
		log.Println("Hello, world!")
	}()

	scanner := bufio.NewScanner(os.Stderr)
	for scanner.Scan() {
		if strings.Contains(scanner.Text(), "Hello, world!") {
			break
		}
	}
}

However, if I run this, it just blocks:

> go run main.go
2022/04/17 00:31:43 Hello, world!

Should this scanner not capture the standard error output and hit the break statement?

If you want to intercept output, you can use a pipe, like so:

r, w, _ := os.Pipe()

log.SetOutput(w)

go func() {
	time.Sleep(time.Second)
	log.Println("Hello, world!")
}()

scanner := bufio.NewScanner(r)

https://go.dev/play/p/HdEs5tbDYDE

It seems that if the io.Reader is os.Stderr, scanner.Scan() blocks indefinitely, whereas if I set it to a custom reader like a *bytes.Buffer, it returns immediately before the Goroutine has a chance to write to it.

Since the context of this logic is a unit test, I worked around this by using assert.Eventually:

func TestScan(t *testing.T) {
	buf := bytes.NewBuffer([]byte{})
	log.SetOutput(buf)

	go func() {
		time.Sleep(time.Second)
		log.Println("Hello, world!")
	}()

	assert.Eventually(t, func() bool {
		scanner := bufio.NewScanner(buf)
		for scanner.Scan() {
			if strings.Contains(scanner.Text(), "Hello, world!") {
				return true
			}
		}
		return false
	}, 10*time.Second, 100*time.Millisecond)
}

This test passes in 1.1 seconds as expected:

> go test ./... -v
=== RUN   TestScan
--- PASS: TestScan (1.10s)
PASS

(Granted, this is not the most efficient solution as it presumably reads the entire output each time, but it should do the job).