fix markup tests

Author: joshgoebel

test/markup/rust/strings.expect.txt

@@ -1,8 +1,8 @@
 <span class="hljs-string">&#x27;a&#x27;</span>;
-<span class="hljs-string">&#x27;\n&#x27;</span>;
-<span class="hljs-string">&#x27;\x1A&#x27;</span>;
-<span class="hljs-string">&#x27;\u12AS&#x27;</span>;
-<span class="hljs-string">&#x27;\U1234ASDF&#x27;</span>;
+<span class="hljs-string">&#x27;<span class="hljs-char escape_">\n</span>&#x27;</span>;
+<span class="hljs-string">&#x27;<span class="hljs-char escape_">\x</span>1A&#x27;</span>;
+<span class="hljs-string">&#x27;<span class="hljs-char escape_">\u</span>12AS&#x27;</span>;
+<span class="hljs-string">&#x27;<span class="hljs-char escape_">\U</span>1234ASDF&#x27;</span>;
 <span class="hljs-string">b&#x27;a&#x27;</span>;
 
 <span class="hljs-string">&quot;hello&quot;</span>;