Followup to r1567492, test both the exact time and near time with pegrevs

* subversion/tests/cmdline/checkout_tests.py
  (checkout_peg_rev_date): Save the output in r1_string and use that to
    run an exact test as well.

Suggested by: danielsh
(See: https://mail-archives.apache.org/mod_mbox/subversion-dev/201307.mbox/%3C20130709135210.GB4075%40lp-shahaf.local%3E)


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1567494 13f79535-47bb-0310-9956-ffa450edef68