org-test: prepare for more selective batch testing

org-test: prepare for more selective batch testing

* testing/org-test.el: Conditionalize interactive setup on
  bound-and-true `org-batch-test'.  (org-id-locations-file): New
  defvar.  (org-test-run-batch-tests): Use `org-id-locations-file'
  defined before and add optional argument `org-test-selector' to
  control which tests to run.

This change introduces ability to select the tests to run for batch
testing and removes several dependencies not needed during batch test
when `org-batch-test' is defined in preparation for a corresponding
change in how the build system invokes batch testing.  Full backward
compatibility is maintained.