Fix recently uncovered 'make check' failures

Fix recently uncovered 'make check' failures

For discussion, see the following thread:
https://lists.gnu.org/r/emacs-devel/2021-01/msg01111.html

* test/lisp/autorevert-tests.el
(auto-revert-test07-auto-revert-several-buffers):
* test/lisp/emacs-lisp/seq-tests.el (test-seq-do-indexed)
(test-seq-random-elt-take-all): Fix errors from using add-to-list on
lexical variables.
* test/lisp/emacs-lisp/cl-lib-tests.el
(cl-lib-defstruct-record): Expect test to succeed when byte-compiled
following change of 2021-01-23 'Fix missing file&line info in
"Unknown defun property" warnings'.
(cl-lib-tests--dummy-function): Remove; no longer needed.
(old-struct): Silence byte-compiler warning about unused lexical
variable.