Indent ERT failure explanations rigidly

Indent ERT failure explanations rigidly

This also affects the listing of `should' forms produced by hitting
the L key on a test button in an ERT buffer.

* lisp/emacs-lisp/ert.el (ert--pp-with-indentation-and-newline):
Indent the pretty-printed result to match the caller's current column
as a reference indentation.
* test/lisp/emacs-lisp/ert-tests.el
(ert--pp-with-indentation-and-newline): New test.  (Bug#72561)

(cherry picked from commit 0a500193087efc96aa3791dc4c2084ef5f6c3c06)