testing: allow to select tests

testing: allow to select tests

* mk/default.mk: Add default for new variable BTEST_RE to select all
  tests.  (BTEST_OB_LANGUAGES): Remove sh (always needed, as
  emacs-lisp) and add comment for ruby.  (BTEST): Use BTEST_RE to
  select tests from the test suite.  Reorganize pre-loading of Org,
  Babel, Ox and babel languages into a known clean environment.

* mk/targets.mk (CONF_TEST): Add BTEST_RE to the output of
  config-test.

* testing/org-batch-test-init.el: New file.  Remove all traces of any
  Org built into Emacs or otherwise present in the environment so it
  can not be picked up spuriously during testing or conceal errors in
  the Org version under test.

* testing/org-test.el (org-test-run-batch-tests): Showtest selection
  in messages.  (org-test-run-all-tests): Update ID locations.