Add some tolerance to lastModified test

Both [Servo](https://github.com/servo/servo/issues/3228) and [Gecko](https://bugzilla.mozilla.org/show_bug.cgi?id=1064625) are getting errors where the test seems to be off by a second.

It seems like a bug in our implementation, but it might also be an issue with the test. This allows for some leeway when it comes to comparing the times.

/cc @Ms2ger