Rework electric-layout-post-self-insert-function (bug#33794)

Rework electric-layout-post-self-insert-function (bug#33794)

This should now fix more problems reported in (bug#33794) regarding
insertion of newlines before and after the opening brace.  Write two
automated tests.

Also provide a new electric-layout-local-mode for testing.

* lisp/electric.el (electric-layout-post-self-insert-function-1):
New function that does the work for
electric-layout-post-self-insert-function-1.
(electric-layout-local-mode): New minor mode.

* test/lisp/electric-tests.el (electric-layout-int-main-kernel-style)
(electric-layout-int-main-allman-style): Add two tests.