* lisp/org.el (org-modules): Remove export back-ends from the list.
Update docstring.
(org-export-backends): New variable.
* lisp/ox.el (org-export-async-start): Make sure export framework is
required in the external process.
* lisp/ob-exp.el (org-babel-exp-process-buffer): Renamed from
`org-export-blocks-preprocess'.
* lisp/ox.el (org-export-execute-babel-code): Apply previous renaming.
* testing/org-test.el (org-test-at-id): Make sure the function returns
the value of the last form in its body.
* testing/lisp/test-ob-exp.el: Fix tests.
* testing/lisp/test-ob-lob.el: Fix tests.