Consolidate redundant checkout error testing code in checkout_tests.py.
Consolidate redundant checkout error testing code in checkout_tests.py.

* subversion/tests/cmdline/checkout_tests.py
  (test_stderr): New helper function.
  (forced_checkout_of_file_with_dir_obstructions,
  forced_checkout_of_dir_with_file_obstructions,
  forced_checkout_with_versioned_obstruction): Eliminate redundant code and
  call new helper instead.

git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@861537 13f79535-47bb-0310-9956-ffa450edef68

wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion