Assume match before calling help-xref-button

Assume match before calling help-xref-button

help-xref-button fails if not preceded by a valid match, so a
preceding unconditional search should not be allowed to fail
silently.

* lisp/emacs-lisp/ert.el (ert-describe-test):
* lisp/help-fns.el (help-fns--compiler-macro)
(help-fns-function-description-header, describe-variable)
(help-fns--customize-variable, describe-face)
(help-fns--face-attributes, describe-keymap): Let unconditional
re-search-backward before help-xref-button fail early, as that would
indicate a logic bug, and the backtrace would more accurately point
to the actual source of the mistake.