ert-x: Move window selection logic to its own macro

ert-x: Move window selection logic to its own macro

* lisp/emacs-lisp/ert-x.el (ert-with-buffer-selected): New macro to
temporarily display a buffer in a selected window and evaluate a body.
(ert-with-test-buffer-selected): Use the new macro.
* test/lisp/whitespace-tests.el
(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): Add tests.
(Bug#60189)