fix: Rewrite ptytest to buffer stdout (#3170)

Fixes #2122

pty/ptytest/ptytest_internal_test.go

@@ -0,0 +1,37 @@
+package ptytest
+
+import (
+	"bytes"
+	"io"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestStdbuf(t *testing.T) {
+	t.Parallel()
+
+	var got bytes.Buffer
+
+	b := newStdbuf()
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		_, err := io.Copy(&got, b)
+		assert.NoError(t, err)
+	}()
+
+	_, err := b.Write([]byte("hello "))
+	require.NoError(t, err)
+	_, err = b.Write([]byte("world\n"))
+	require.NoError(t, err)
+	_, err = b.Write([]byte("bye\n"))
+	require.NoError(t, err)
+
+	err = b.Close()
+	require.NoError(t, err)
+	<-done
+
+	assert.Equal(t, "hello world\nbye\n", got.String())
+}