**Input:** None.

**Goal:** Return the tensor product of the following two matrices:

$$\begin{bmatrix} 1 & 2 \\ 3 & 4 \end{bmatrix} \otimes \begin{bmatrix} 5 & 6 \\ 7 & 8 \end{bmatrix}$$

> In this task, we're using real-valued matrices represented as Q# `Double[][]` type again for simplicity.
