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

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 f2173ae38fa49235c3cdc28ae2ca2e19a375a596)

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