Changes to checkpoint04 to remove or debug failures on Windows

[2020/04/20 05:34:50.251] ======================================================================

[2020/04/20 05:34:50.251] FAIL: test_checkpoint04.test_checkpoint04.test_checkpoint_stats

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[2020/04/20 05:34:50.251] Traceback (most recent call last):

[2020/04/20 05:34:50.251]   File "C:\data\mci\a18701416e890f070f659cc98d7f0906\wiredtiger\test\suite\test_checkpoint04.py", line 98, in test_checkpoint_stats

[2020/04/20 05:34:50.251]     self.get_stat(stat.conn.txn_checkpoint_time_min))

[2020/04/20 05:34:50.251] AssertionError: 312 not less than 203

[2020/04/20 05:34:50.251] ----------------------------------------------------------------------