tw := &teststringwriter{}
		b := NewWriterSize(tw, BufSize)
		b.WriteString("123456789")   // long string, empty buffer:
		tw.check(t, "", "123456789") // use WriteString
	}
	{
		tw := &teststringwriter{}
		b := NewWriterSize(tw, BufSize)
		b.WriteString("abc")
		tw.check(t, "", "")
		b.WriteString("123456789012345")      // long string, non-empty buffer