Add "Testing Org" section

README.org

@@ -2306,9 +2306,8 @@ list detailing and motivating each listing:
    [[info:org#Hyperlinks][hyperlinks (info)]].
 5. Listing [[lst:setup-org-mode-map]] extends the =org-mode-map= with useful
    key-bindings.
-6. Listing [[lst:setup-org-src]] facilitates [[info:org#Editing Source Code][editing Python source code blocks]],
-   and it provides a function to remove the indentation of all =org-src-mode=
-   blocks without =-i= switch.
+6. Listing [[lst:setup-org-src]] facilitates [[info:org#Editing Source Code][editing source code blocks]], and it
+   provides functions to change the indentation of all =org-src-mode= blocks.
 7. Listing [[lst:setup-ob-python]] allows to pretty-print Python session source
    block values with [[https://github.com/psf/black#readme][black]] instead of [[https://docs.python.org/3/library/pprint.html][pprint]].  This snippet may break in the
    future, because it sets =org-babel-python--def-format-value= which is a
@@ -2504,6 +2503,21 @@ When called twice, replace the previously inserted \\(\\) by one $."
             (org-src-preserve-indentation nil))
         (org-indent-block))))
 
+  (defun org-right-shift-block (&optional arg)
+    "Right-shift the block at point 4 spaces.
+With prefix argument ARG, prompt for the number of spaces."
+    (interactive "P")
+    (let ((spaces 4)
+          (start (org-babel-where-is-src-block-head)))
+      (if (not start) (message "Not at block")
+        (when arg
+          (setq spaces (read-number "Number of spaces: ")))
+        ;; The `SUBEXP' of the block body is 5.
+        (goto-char (match-end 5))
+        (forward-line -1)
+        (string-insert-rectangle
+         (match-beginning 5) (point) (make-string spaces ?\s)))))
+
   (declare-function org-babel-map-src-blocks "ext:ob-core" (file &rest body))
   (declare-function org-do-remove-indentation "ext:org-macs" (n skip-fl))
 
@@ -3671,6 +3685,97 @@ define the [[https://orgmode.org/][Org mode]] =kbd= macro in listing
      (latex (format "@@latex:\\colorbox{PowderBlue}{\\texttt{%s}}@@" keys)))))
 #+end_src
 
+*** [[https://orgmode.org/worg/org-tests/index.html][Testing Org]]
+:PROPERTIES:
+:CUSTOM_ID: sec:testing-org
+:END:
+
+Listing [[lst:setup-org-mode-test-1]] and [[lst:setup-org-mode-test-2]] provide
+facilities for [[https://orgmode.org/worg/org-tests/index.html][Testing Org]].
+
+#+caption[Testing Org facilities 1]:
+#+caption: Testing Org facilities 1.
+#+name: lst:setup-org-mode-test-1
+#+begin_src emacs-lisp -n
+(with-eval-after-load 'emacs
+  (defvar org-testing-dir nil)
+  (defvar org-testing-lisp-files nil)
+
+  (defun setup-org-mode-test ()
+    "Intialize an `org-mode' test."
+    (interactive)
+    (unless (and org-testing-dir org-testing-lisp-files)
+      (find-library "org.el")
+      (quit-window)
+      (setq org-testing-dir
+            (file-name-concat
+             (file-name-directory
+              (directory-file-name
+               (file-name-directory
+                (buffer-file-name (get-buffer "org.el")))))
+             (file-name-as-directory "testing")))
+      (message "`%s'" org-testing-dir)
+      (setq org-testing-lisp-files
+            (seq-filter
+             (lambda (f)
+               (string-suffix-p ".el" f))
+             (directory-files
+              (file-name-concat org-testing-dir
+                                (file-name-as-directory "lisp")))))
+      (message "`%s'" org-testing-lisp-files))
+    (unless (fboundp 'org-test-with-temp-text)
+      (find-file (file-name-concat org-testing-dir "org-test.el"))
+      (eval-buffer)
+      (quit-window))
+    (let* ((test
+            (completing-read "Org-mode test: "
+                             org-testing-lisp-files nil 'require-match))
+           (file ))
+      (find-file (file-name-concat
+                  org-testing-dir (file-name-as-directory "lisp") test))
+      (eval-buffer)
+      (message "Evaluated '%s'"
+               (file-name-nondirectory (buffer-file-name (get-buffer test))))
+      (quit-window))))
+#+end_src
+
+#+caption[Testing Org facilities 2]:
+#+caption: Testing Org facilities 2.
+#+name: lst:setup-org-mode-test-2
+#+begin_src emacs-lisp -n
+(with-eval-after-load 'emacs
+  (defun org-test-run-some-tests ()
+    "Run some defined tests.
+Load all test files first."
+    (interactive)
+    (org-test-touch-all-examples)
+    (org-test-update-id-locations)
+    (org-test-load)
+    (let (;; Catch errors in diary sexps better.
+          (calendar-debug-sexp t))
+      ;; test-org-element/normalize-contents fails (can't fix).
+      ;; test-ob-exp/noweb-on-export fails due to spacing (can't fix).
+      ;; test-ob-exp/noweb-on-export-with-exports-results due to spacing (idem).
+      ;; ob-maxima fixed but spacing is fragile. Which Maxima?
+      ;; ob-python hangs sometimes, but very seldom.
+      ;; ob-tangle/continued-code-blocks-w-noweb-ref fails.
+      (ert (rx (or "org/" "org-fold" "org-element" "org-macro" "org-src"
+                   "property-inheritance"
+                   "ob/" "ob-emacs-lisp" "ob-eshell" "ob-maxima"
+                   "ob-python" "ob-tangle")))
+      (org-test-kill-all-examples)))
+
+  (defun org-test-run-ob-tests ()
+    "Run all \"ob\" tests.
+Load all test files first."
+    (interactive)
+    (org-test-load)
+    (let (;; Catch errors in diary sexps better.
+          (calendar-debug-sexp t))
+      (ert "ob/"))))
+#+end_src
+
+
 *** [[info:org#Export Settings][File inclusion (info)]] and [[info:org#Noweb Reference Syntax][Noweb (info)]] trickery
 :PROPERTIES:
 :CUSTOM_ID: sec:file-inclusion-and-noweb