emacs: add invisible space after the search widget field in notmuch-hello

emacs: add invisible space after the search widget field in notmuch-hello

It is very convenient when C-e (bound to `widget-end-of-line') ignores
trailing spaces inside the search widget.  But it only does so if a
widget is not followed by a newline (that is why it works in the saved
search widgets).  The patch just adds an invisible space after the
search widget to get the desirable behavior of `widget-end-of-line'.
The extra space is also added to expected results of emacs tests.