test_util: fix skip-on-32bit-platform behavior

I set up a raspberry pi buildslave (which, on the "raspbian jesse"
image, uses a 32-bit python, and perhaps a 32-bit kernel too). It fails
test_util.TimeFormat.test_format_time_y2038 with a ValueError inside the
call to time.gmtime(). The test was looking for the equality check to
fail instead. I think catching ValueError is the more-correct way to
detect a system with a 32-bit time type.

src/allmydata/test/test_util.py

@@ -1047,10 +1047,14 @@ class TimeFormat(unittest.TestCase, TimezoneMixin):
     def test_format_time_y2038(self):
         seconds_per_day = 60*60*24
         leap_years_1970_to_2047_inclusive = ((2044 - 1968) // 4)
+        t = (seconds_per_day*
+             ((2048 - 1970)*365 + leap_years_1970_to_2047_inclusive))
         try:
-            self.failUnlessEqual(time_format.format_time(time.gmtime(seconds_per_day*((2048 - 1970)*365+leap_years_1970_to_2047_inclusive))), '2048-01-01 00:00:00')
-        except unittest.FailTest:
+            gm_t = time.gmtime(t)
+        except ValueError:
             raise unittest.SkipTest("Note: this system cannot handle dates after 2037.")
+        self.failUnlessEqual(time_format.format_time(gm_t),
+                             '2048-01-01 00:00:00')
 
     def test_format_delta(self):
         time_1 = 1389812723