readTo := func(delim byte, want string) {
			data, err := read(r, delim)
			if err != nil {
				t.Fatalf("#%d: unexpected error reading to %c: %v", rno, delim, err)
			}
			if got := string(data); got != want {
				t.Fatalf("#%d: got %q, want %q", rno, got, want)
			}
		}

		// Read the data with occasional UnreadByte calls.
		for i := 0; i < n; i++ {
			readTo('d', "abcd")
			for j := 0; j < 3; j++ {