Move buffer related functions from ert-x.el to ert.el

Move buffer related functions from ert-x.el to ert.el

* lisp/emacs-lisp/ert-x.el (ert--text-button)
(ert--format-test-buffer-name, ert--test-buffers)
(ert--test-buffer-button, ert--test-buffer-button-action)
(ert--call-with-test-buffer, ert-with-test-buffer)
(ert-with-buffer-selected, ert-kill-all-test-buffers)
(ert-call-with-buffer-renamed, ert-buffer-string-reindented): Move
from here...
* lisp/emacs-lisp/ert.el (ert--text-button)
(ert--format-test-buffer-name, ert--test-buffers)
(ert--test-buffer-button, ert--test-buffer-button-action)
(ert--call-with-test-buffer, ert-with-test-buffer)
(ert-kill-all-test-buffers, ert-with-buffer-selected)
(ert-call-with-buffer-renamed, ert-with-buffer-renamed): ...to
here.
* doc/misc/ert.texi (Helpers for Buffers): Break out new section...
(Helper Functions): ...from here.
* lisp/emacs-lisp/ert-x.el (ert-with-test-buffer-selected): Move
obsolete definition to the end of the file.

* test/lisp/emacs-lisp/ert-x-tests.el
(ert--hash-table-to-alist, ert-test-test-buffers)
(ert-test-with-buffer-selected/current)
(ert-test-with-buffer-selected/selected)
(ert-test-with-buffer-selected/nil-buffer)
(ert-test-with-buffer-selected/modification-hooks)
(ert-test-with-buffer-selected/read-only)
(ert-test-with-buffer-selected/return-value)
(ert-test-with-test-buffer-selected/modification-hooks)
(ert-test-with-test-buffer-selected/read-only)
(ert-test-with-test-buffer-selected/return-value)
(ert-test-with-test-buffer-selected/buffer-name): Move tests from
here...
* test/lisp/emacs-lisp/ert-tests.el
(ert--hash-table-to-alist, ert-test-test-buffers)
(ert-test-with-buffer-selected/current)
(ert-test-with-buffer-selected/selected)
(ert-test-with-buffer-selected/nil-buffer)
(ert-test-with-buffer-selected/modification-hooks)
(ert-test-with-buffer-selected/read-only)
(ert-test-with-buffer-selected/return-value)
(ert-test-with-test-buffer-selected/selected)
(ert-test-with-test-buffer-selected/modification-hooks)
(ert-test-with-test-buffer-selected/read-only)
(ert-test-with-test-buffer-selected/return-value)
(ert-test-with-test-buffer-selected/buffer-name): ...to here.
* test/lisp/progmodes/hideshow-tests.el (ert-x):
* test/lisp/simple-tests.el (ert-x):
* test/lisp/whitespace-tests.el (ert-x): Don't require.

(cherry picked from commit c9e681aa0c75feaf1c0a5495b0d475698cbdb653)