fix: Increase ptytest buffer to resolve flake (#1141)

From investigating the following run:
https://github.com/coder/coder/runs/6156348454?check_suite_focus=true

It's believed that the cause is data being discarded due to the buffer
filling up. This _might_ fix it, but not entirely sure.

agent/agent_test.go

@@ -76,6 +76,11 @@ func TestAgent(t *testing.T) {
 		session.Stdin = ptty.Input()
 		err = session.Start(command)
 		require.NoError(t, err)
+		caret := "$"
+		if runtime.GOOS == "windows" {
+			caret = ">"
+		}
+		ptty.ExpectMatch(caret)
 		ptty.WriteLine("echo test")
 		ptty.ExpectMatch("test")
 		ptty.WriteLine("exit")

pty/ptytest/ptytest.go

@@ -3,6 +3,7 @@ package ptytest
 import (
 	"bufio"
 	"bytes"
+	"context"
 	"io"
 	"os"
 	"os/exec"
@@ -10,6 +11,7 @@ import (
 	"runtime"
 	"strings"
 	"testing"
+	"time"
 	"unicode/utf8"
 
 	"github.com/stretchr/testify/require"
@@ -76,6 +78,19 @@ func (p *PTY) ExpectMatch(str string) string {
 	var buffer bytes.Buffer
 	multiWriter := io.MultiWriter(&buffer, p.outputWriter)
 	runeWriter := bufio.NewWriterSize(multiWriter, utf8.UTFMax)
+	complete, cancelFunc := context.WithCancel(context.Background())
+	defer cancelFunc()
+	go func() {
+		timer := time.NewTimer(10 * time.Second)
+		defer timer.Stop()
+		select {
+		case <-complete.Done():
+			return
+		case <-timer.C:
+		}
+		_ = p.Close()
+		p.t.Errorf("match exceeded deadline: wanted %q; got %q", str, buffer.String())
+	}()
 	for {
 		var r rune
 		r, _, err := p.runeReader.ReadRune()