Improve failure reporting in test/lisp/electric-tests.el

Improve failure reporting in test/lisp/electric-tests.el

In particular, on a failure, output the test's doc string to
electric-tests.log, along with all the other failure information.
Fixes bug #47320.

* electric-tests.el (electric-pair-test-for) New parameter doc-string.  On a
test failure, output the doc-string parameter with message.
(electric-pair-define-test-form): Set the new variable doc-string to the
generated doc string, and pass this as argument to both ert-deftest and
electric-pair-test-for.