[monotouch-test] Fix regression in CalendarTest to not compare UTC and local time directly. (#2964)

In 0442cdf9c0 the `now` variable was changed to
be a UTC date, but unfortunately the code that compares it to a local date
(NSDate.Now) wasn't updated.

Only year/month/day values were compared, which meant the test would fail if
run when UTC and local time didn't represent the same date (and conversely
would pass if the UTC and local date was the same date, which is why the
changed did not fail the PR test run: the PR was tested during the 19 hours of
the day when EST and UTC represent the sam date).

Fix this by converting the UTC `now` to NSDate instead of using NSDate.Now.

This has the additional benefit of also fixing a (much smaller) race
condition: if midnight occurred just between calculating `now` and NSDate.Now,
the test would also fail.

tests/monotouch-test/Foundation/CalendarTest.cs

@@ -37,7 +37,7 @@ namespace MonoTouchFixtures.Foundation {
 			var now = DateTime.Now.ToUniversalTime ();
 			NSDateComponents comps;
 
-			comps = cal.Components (NSCalendarUnit.Year | NSCalendarUnit.Month | NSCalendarUnit.Day, NSDate.Now);
+			comps = cal.Components (NSCalendarUnit.Year | NSCalendarUnit.Month | NSCalendarUnit.Day, (NSDate) now);
 			Assert.AreEqual (now.Year, comps.Year, "a year");
 			Assert.AreEqual (now.Month, comps.Month, "a month");
 			Assert.AreEqual (now.Day, comps.Day, "a day");