Fix test_timestamp20 occasional failure by doing a checkpoint before evicting the pages

Python Stacktrace

FAIL: test_timestamp20.test_timestamp20.test_timestamp20_modify(column) (subunit.RemotedTestCase)

test_timestamp20.test_timestamp20.test_timestamp20_modify(column)

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testtools.testresult.real._StringException: Traceback (most recent call last):

  File "/data/mci/739795a224aeb15343cabcdebd00d46e/wiredtiger/test/suite/wttest.py", line 328, in _callTestMethod

    method()

  File "/data/mci/739795a224aeb15343cabcdebd00d46e/wiredtiger/test/suite/test_timestamp20.py", line 201, in test_timestamp20_modify

    self.assertEqual(old_reader_cursor[self.get_key(i)], expected)

  File "/opt/mongodbtoolchain/revisions/c6da1cf7f0b4b60d53566305e59857d3d540dcf7/stow/python3-v3.fZa/lib/python3.9/unittest/case.py", line 831, in assertEqual

    assertion_func(first, second, msg=msg)

  File "/opt/mongodbtoolchain/revisions/c6da1cf7f0b4b60d53566305e59857d3d540dcf7/stow/python3-v3.fZa/lib/python3.9/unittest/case.py", line 1211, in assertMultiLineEqual

    self.fail(self._formatMessage(msg, standardMsg))

  File "/opt/mongodbtoolchain/revisions/c6da1cf7f0b4b60d53566305e59857d3d540dcf7/stow/python3-v3.fZa/lib/python3.9/unittest/case.py", line 670, in fail

    raise self.failureException(msg)

AssertionError: 'aaaa[191 chars]aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[255 chars]aaaa' != 'aaaa[191 chars]aaaaaCaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[255 chars]aaaa'

Diff is 1415 characters long. Set self.maxDiff to None to see it.

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Ran 29136 tests in 1897.340s

Link: https://evergreen.mongodb.com/task_log_raw/wiredtiger_rhel80_unit_test_long_2f0657608187ffa9231a01f034d2f6cbde5dba5c_22_06_06_06_02_00/0?type=T#L93840

Evergreen YAML Script Failure

Command failed: shell script encountered problem: exit code 1

Link: https://evergreen.mongodb.com/task_log_raw/wiredtiger_rhel80_unit_test_long_2f0657608187ffa9231a01f034d2f6cbde5dba5c_22_06_06_06_02_00/0?type=T#L93859