Fix file size comparison failure

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[2021/03/23 01:15:41.534] FAIL: test_backup03.test_backup_target.test_backup_target -s 10 (backup_7c)

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[2021/03/23 01:15:41.534] Traceback (most recent call last):

[2021/03/23 01:15:41.534]   File "/data/mci/22e27b7a0d3492ac1521e7c8012cf56e/wiredtiger/test/suite/wttest.py", line 483, in tearDown

[2021/03/23 01:15:41.534]     self.captureout.check(self)

[2021/03/23 01:15:41.534]   File "/data/mci/22e27b7a0d3492ac1521e7c8012cf56e/wiredtiger/test/suite/wttest.py", line 116, in check

[2021/03/23 01:15:41.534]     testcase.fail('unexpected ' + self.desc + ', contains: "' +

[2021/03/23 01:15:41.534] AssertionError: unexpected standard output, contains: "file comparison failed: ./backup_base size 4121572 != ./backup_other size 4121653

[2021/03/23 01:15:41.534] "

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