bpo-35066: Make trailing percent test more portable. (GH-15907)

Different libc implementations have different behavior when presented with trailing % in strftime strings. To make test_strftime_trailing_percent more portable, compare the output of datetime.strftime directly to that of time.strftime rather than hardcoding.
(cherry picked from commit f2173ae38f)

Co-authored-by: Benjamin Peterson <benjamin@python.org>

Lib/test/datetimetester.py

@@ -1449,15 +1449,20 @@ class TestDate(HarmlessMixedComparison, unittest.TestCase):
         t.strftime("%f")
 
     def test_strftime_trailing_percent(self):
-        # bpo-35066: make sure trailing '%' doesn't cause
-        # datetime's strftime to complain
+        # bpo-35066: Make sure trailing '%' doesn't cause datetime's strftime to
+        # complain. Different libcs have different handling of trailing
+        # percents, so we simply check datetime's strftime acts the same as
+        # time.strftime.
         t = self.theclass(2005, 3, 2)
         try:
             _time.strftime('%')
         except ValueError:
             self.skipTest('time module does not support trailing %')
-        self.assertEqual(t.strftime('%'), '%')
-        self.assertEqual(t.strftime("m:%m d:%d y:%y %"), "m:03 d:02 y:05 %")
+        self.assertEqual(t.strftime('%'), _time.strftime('%', t.timetuple()))
+        self.assertEqual(
+            t.strftime("m:%m d:%d y:%y %"),
+            _time.strftime("m:03 d:02 y:05 %", t.timetuple()),
+        )
 
     def test_format(self):
         dt = self.theclass(2007, 9, 10)