Test case test_timer_stop_on_save fails occasionally

[cdubz]

The core.tests.tests_forms.InitialValuesTestCase.test_timer_stop_on_save test case fails inconsistently because of small datetime differences. E.g. --

[14:16:13] Starting 'coverage'...
Creating test database for alias 'default'...
System check identified no issues (0 silenced).
.....................................................F...................................................................
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FAIL: test_timer_stop_on_save (core.tests.tests_forms.InitialValuesTestCase)
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Traceback (most recent call last):
  File "/home/travis/build/cdubz/babybuddy/core/tests/tests_forms.py", line 138, in test_timer_stop_on_save
    self.assertEqual(self.localtime_string(self.timer.end), params['end'])
AssertionError: '02/05/2020 02:16:28 PM' != '02/05/2020 02:16:27 PM'
- 02/05/2020 02:16:28 PM
?                   ^
+ 02/05/2020 02:16:27 PM
?                   ^
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Ran 121 tests in 29.042s
FAILED (failures=1)
Destroying test database for alias 'default'...
[14:16:46] 'coverage' errored after 33 s