Decouple require-theme from load-theme

Decouple require-theme from load-theme

* lisp/custom.el (require-theme): Refashion after 'require', as a
function for loading only named features.  Do not call
load-theme (bug#45068).
* etc/NEWS: Update its announcement accordingly.
* doc/lispref/customize.texi (Custom Themes): Document it.

* etc/themes/modus-operandi-theme.el:
* etc/themes/modus-vivendi-theme.el: Remove redundant calls to
'provide'.

* test/lisp/custom-tests.el (custom-tests--with-temp-dir): New
macro.
(custom-theme--load-path): Use it.
(custom-tests-require-theme): New test.