[corlib] Bump allowed diff time in FileTest.LastWriteTimeSubMs

I've seen it fail with this:

```
1) LastWriteTimeSubMs (MonoTests.System.IO.FileTest.LastWriteTimeSubMs)
     Iteration #22 failed, diff.TotalSeconds: 1.000023, untouched: 20:14:03:0000000, now: 20:14:04:0000230
  Expected: True
  But was:  False
```

mcs/class/corlib/Test/System.IO/FileTest.cs

@@ -1279,7 +1279,7 @@ public void LastWriteTimeSubMs ()
 					var now = DateTime.UtcNow;
 					var diff = now - untouched;
 					// sanity check
-					Assert.IsTrue (diff.TotalSeconds >= 0 && diff.TotalSeconds < 1.0, $"Iteration #{i} failed, diff.TotalSeconds: {diff.TotalSeconds}, untouched: {untouched.ToString (fmt)}, now: {now.ToString (fmt)}");
+					Assert.IsTrue (diff.TotalSeconds >= 0 && diff.TotalSeconds < 2.0, $"Iteration #{i} failed, diff.TotalSeconds: {diff.TotalSeconds}, untouched: {untouched.ToString (fmt)}, now: {now.ToString (fmt)}");
 					File.SetLastWriteTimeUtc (path, now);
 					var touched = File.GetLastWriteTimeUtc (path);