tests: Remove redundant xzdec.js include (#676)

Follows-up 0c53010, which removed this from www/index.html,
but it was still present in the test suite.

This is now loaded automatically by RequireJS.

tests.html

@@ -31,7 +31,6 @@
              js files. This loads "main.js", which in turn can load other
              files, all handled by require.js:
              http://requirejs.org/docs/api.html#jsfiles -->
-        <script type="text/javascript" src="www/js/lib/xzdec.js"></script>
         <script type="text/javascript"
                 data-main="tests/init.js"
         src="www/js/lib/require.js"></script>