Don't bind search-spaces-regexp around possible autoload (Bug#35802)

Don't bind search-spaces-regexp around possible autoload (Bug#35802)

* lisp/isearch.el (isearch-search-fun-default): Move possible autoload
trigger outside let-binding of search-spaces-regexp.
* lisp/char-fold.el (char-fold-make-table): Remove no longer needed
workaround.

* lisp/info.el (Info-search-whitespace-regexp):
* lisp/isearch.el (search-whitespace-regexp):
* src/search.c (syms_of_search) <search-spaces-regexp>: Add warning
about adding capturing groups to the value.

* test/lisp/char-fold-tests.el: Remove, binding search-spaces-regexp
to a different should be considered a bug.