Stop Testcover from producing spurious 1value errors

Stop Testcover from producing spurious 1value errors

Fix bug#25351 by copying results of form evaluations for later
comparison.
* lisp/emacs-lisp/testcover.el (testcover-after): Copy the result
of a form's first evaluation and compare subsequent evaluations to
the copy. Improve the error message used when a form's value
changes.
(testcover--copy-object, testcover--copy-object1): New functions.
* test/lisp/emacs-lisp/testcover-resources/testcases.el
(by-value-vs-by-reference-bug-25351): Remove expected failure tag.
(circular-lists-bug-24402): Add another circular list case.