Fix completion boundaries for TRAMP file names

Fix completion boundaries for TRAMP file names

Previously, we assumed (roughly) that substitute-in-file-name
always returns a suffix of the original string.  But
substitute-in-file-name on "/ssh:user@host:/~/" returns
"/ssh:user@host:~/", preserving the TRAMP magic prefix.  Weaken
the assertion in completion--sifn-boundaries to allow this; the
new assertion is more clear about the property we care about,
anyway.

* lisp/minibuffer.el (completion--sifn-boundaries): Weaken
assertion slightly.

* test/lisp/net/tramp-tests.el (tramp--test-emacs31-p)
(tramp--split-on-boundary)
(tramp-test51-file-name-completion-boundaries): Add.

(cherry picked from commit d56d7ca35cf3c0185fe42bf7f64e3aa6a5281e3e)