Fix Testcover bug in handling of vectors containing dotted lists

Fix Testcover bug in handling of vectors containing dotted lists

* lisp/emacs-lisp/testcover.el (testcover-analyze-coverage-compose):
Handle dotted lists. Fix bug#30909.
* test/lisp/emacs-lisp/testcover-resources/testcases.el:
(dotted-list-in-vector-bug-30909): New test case.
(quotes-within-backquotes-bug-25316, dotted-backquote)
(quoted-backquote, backquoted-vector-bug-25316)
(vector-in-macro-spec-bug, backquoted-dotted-alist): Change
docstrings to mention analyzing code instead of reinstrumenting
it.