Fix ert-test finding by symbol (Bug#28849)

Fix ert-test finding by symbol (Bug#28849)

In 2017-07-28 "* lisp/subr.el (define-symbol-prop): New function", the
symbol used for ert tests in load-history was changed from
`ert-deftest' to `ert--test', but a couple of other places were not
updated accordingly.
* lisp/emacs-lisp/ert.el (ert-find-test-other-window): Pass
`ert--test' as the TYPE to `find-function-do-it'.
(top-level): Use `ert--test' as the key to the
`find-function-regexp-alist' entry.