Update tests

test/latexify_refs/integral1.txt

@@ -1,3 +1,3 @@
 \begin{equation}
-\int_{0}^{1)} \texttt{dx} x
+\int_{0}^{1)} \mathtt{dx} x
 \end{equation}

test/latexify_refs/integral2.txt

@@ -1,3 +1,3 @@
 \begin{equation}
-\int_{-\infty}^{\infty)} \texttt{dx} \left( u\left( x \right) \right)^{2}
+\int_{-\infty}^{\infty)} \mathtt{dx} \left( u\left( x \right) \right)^{2}
 \end{equation}

test/latexify_refs/integral3.txt

@@ -1,3 +1,3 @@
 \begin{equation}
-\int_{ - z}^{u\left( x \right))} \texttt{dx} x^{2}
+\int_{ - z}^{u\left( x \right))} \mathtt{dx} x^{2}
 \end{equation}