Test and fix "/:" quoted file name handlers (Bug#30243)

Test and fix "/:" quoted file name handlers (Bug#30243)

* lisp/files.el (file-name-non-special): Strip the "/:" from
`default-directory' for `temporary-file-directory' operation; both
arguments to `file-name-completion', `file-name-all-completion', and
`file-equal-p' operations; `buffer-file-name' for
`make-auto-save-file-name' and 'set-visited-file-modtime' operations.
Don't touch any operands of `file-notify-rm-watch' and
`file-notify-valid-p' as they receive descriptors; not file
names (this is not sufficient to fix these operations for "/:" quoted
file names though).
* test/lisp/files-tests.el (files-tests--with-temp-dir): New macro.
(files-file-name-non-special-notify-handlers)
(files-file-name-non-special-handlers): New tests.