test: add failing expression ignore test

Author: cossssmin

test/expect/expression_ignored.html

@@ -2,3 +2,7 @@
 <p data-username="{{ user.name }}" data-user-id="user-{{ user.id }}-bar-{{ foo }}">
   Here's one {{ variable }} and here's {{ another }}. And some bar.
 </p>
+
+ignored: {{ foo }}
+ignoredUnescaped: {{{ foo }}}
+rendered: bar

test/fixtures/expression_ignored.html

@@ -2,3 +2,7 @@
 <p data-username="@{{ user.name }}" data-user-id="user-@{{ user.id }}-{{ foo }}-@{{ foo }}">
   Here's one @{{ variable }} and here's @{{ another }}. And some {{ foo }}.
 </p>
+
+ignored: @{{ foo }}
+ignoredUnescaped: @{{{ foo }}}
+rendered: {{ foo }}