time: add leap year test for Date

I'm writing a matching implementation of the time package and missed
the "add one day in a leap year" block. This test would have caught my
error.

I understand we can't add test cases for every Date but it seems like
"tripped up someone attempting to reimplement this" is a good
indicator it may trip up people in the future.

Change-Id: I4c3b51e52e269215ec0e52199afe604482326edb
Reviewed-on: https://go-review.googlesource.com/56490Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/time/time_test.go

@@ -575,6 +575,7 @@ var dateTests = []struct {
 	{2011, 3, 13, 1, 59, 59, 0, Local, 1300010399}, // 1:59:59 PST
 	{2011, 3, 13, 3, 0, 0, 0, Local, 1300010400},   // 3:00:00 PDT
 	{2011, 3, 13, 2, 30, 0, 0, Local, 1300008600},  // 2:30:00 PDT ≡ 1:30 PST
+	{2012, 12, 24, 0, 0, 0, 0, Local, 1356336000},  // Leap year
 
 	// Many names for Fri Nov 18 7:56:35 PST 2011
 	{2011, 11, 18, 7, 56, 35, 0, Local, 1321631795},                 // Nov 18 7:56:35