Revert test pass symbol in List reporter

When the reporter symbols were abstracted into the Base module (bec7a6e), the List reporter was accidentally changed to show a dot when a test passes, instead of a checkmark like it used to. The old, correct behavior is also shown on the homepage: https://mochajs.org/#list.

lib/reporters/list.js

@@ -42,7 +42,7 @@ function List (runner) {
   });
 
   runner.on('pass', function (test) {
-    var fmt = color('checkmark', '  ' + Base.symbols.dot) +
+    var fmt = color('checkmark', '  ' + Base.symbols.ok) +
       color('pass', ' %s: ') +
       color(test.speed, '%dms');
     cursor.CR();