Add characterization test for sanitize-html

At various times in the past, upgrades in the sanitize-html library that
changed behavior of HTML filtering have caused things like emotes to
break unexpectedly.  This commit adds a basic test to sanitize
non-alphanumeric characters found in channels' emote codes so that if
the library changes, the test will break and give a heads up that
something changed.

test/xss.js

@@ -0,0 +1,12 @@
+const assert = require('assert');
+const XSS = require('../lib/xss');
+
+describe('XSS', () => {
+    describe('sanitizeHTML', () => {
+        it('behaves consistently w.r.t. special chars used in emotes', () => {
+            const input    = '`^~=| _-,;:!?/."()[]{}@$*\\&#%+á\t';
+            const expected = '`^~=| _-,;:!?/."()[]{}@$*\\\\&#%+á\t';
+            assert.strictEqual(XSS.sanitizeHTML(input), expected);
+        });
+    });
+});