Fix comint-password-prompt-regexp

Fix comint-password-prompt-regexp

The change from 2017-12-22 "Support French password prompts in shell"
tried to allow nonbreaking space in addition to whitespace syntax
characters around the colon, but used square brackets which cause "\s"
to be interpreted literally rather than as a backslash construct.
* lisp/comint.el (comint-password-prompt-regexp): Use [[:blank:]]
instead, which also has the benefit of not relying on the major mode's
whitespace syntax setting.
* test/lisp/comint-tests.el (comint-testsuite-password-strings):
Update French localized entry to have a space before the colon, as
reported in Bug#29729.