Fix bug in how ERT invokes its debugger.

Fix bug in how ERT invokes its debugger.

The debugger needs to receive a list of the error symbol and data;
cf. the documentation of the `debugger' variable.  This bug manifested
itself in ERT forms such as (should (integerp (ert-fail "Boo"))),
which resulted in an incorrect condition object.  Note that forms such
as (should (ert-fail "Boo")) weren't affected because they wouldn't
use the `ert--should-signal-hook'.

* test/emacs-lisp/ert.el (ert--should-signal-hook): Call debugger with
the right arguments.

* test/lisp/emacs-lisp/ert-tests.el (ert-test-fail-inside-should): Add
unit test.