test(cli): improve TestServer/SpammyLogs line count (#16814)

pty/ptytest/ptytest.go

@@ -319,6 +319,11 @@ func (e *outExpecter) ReadLine(ctx context.Context) string {
 	return buffer.String()
 }
 
+func (e *outExpecter) ReadAll() []byte {
+	e.t.Helper()
+	return e.out.ReadAll()
+}
+
 func (e *outExpecter) doMatchWithDeadline(ctx context.Context, name string, fn func(*bufio.Reader) error) error {
 	e.t.Helper()
 
@@ -460,6 +465,18 @@ func newStdbuf() *stdbuf {
 	return &stdbuf{more: make(chan struct{}, 1)}
 }
 
+func (b *stdbuf) ReadAll() []byte {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	if b.err != nil {
+		return nil
+	}
+	p := append([]byte(nil), b.b...)
+	b.b = b.b[len(b.b):]
+	return p
+}
+
 func (b *stdbuf) Read(p []byte) (int, error) {
 	if b.r == nil {
 		return b.readOrWaitForMore(p)