Tweak electric-layout-mode's API again after Stefan's comments

Tweak electric-layout-mode's API again after Stefan's comments

* lisp/electric.el (electric-layout-rules): Tweak docstring.
(electric-layout-post-self-insert-function-1): MATCHER is always a
char.  Call function with last-command-event.

* test/lisp/electric-tests.el (electric-pair-mode-newline-between-parens)
(electric-layout-mode-newline-between-parens-without-e-p-m): Don't
pass a name to the ERT test buffer.
(electric-layout-mode-newline-between-parens-without-e-p-m-2): New test.