FIX: Don't fail if the test environment doesn't support Webauthn

test/javascripts/acceptance/preferences-test.js.es6

@@ -256,6 +256,10 @@ QUnit.test("second factor security keys", async assert => {
   assert.ok(exists("#security-key-name"), "shows security key name input");
 
   fillIn("#security-key-name", "");
+
+  // The following tests can only run when Webauthn is enabled. This is not
+  // always the case, for example on a browser running on a non-standard port
+  if (typeof PublicKeyCredential !== "undefined") {
     await click(".add-security-key");
 
     assert.ok(
@@ -264,6 +268,7 @@ QUnit.test("second factor security keys", async assert => {
         .indexOf("provide a name") > -1,
       "shows name missing error message"
     );
+  }
 });
 
 QUnit.test("default avatar selector", async assert => {