Make I/O tests use separate working sets.

Instead of tracking current state to globally shared test descriptions,
we use them only as a template and provide each test with a separate copy.
Also, store files in test-specific folders.

* subversion/tests/libsvn_subr/io-test.c
  (TEST_DIR): rename to ...
  (TEST_DIR_PREFIX): ... this
  (test_file_definitions): rename to ...
  (test_file_definitions_template): ... this
  (create_test_file): write to a test-specific folder
  (create_comparison_candidates): provide a copy of the test data and
                                  create files in test-specific folders
  (test_two_file_size_comparison,
   test_two_file_content_comparison,
   test_three_file_size_comparison, 
   test_three_file_content_comparison): update callers

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1544729 13f79535-47bb-0310-9956-ffa450edef68