Enabling code coverage on a file containing an ert test that uses should-error causes the test to fail.

Steps to reproduce:
1. Start emacs (I used: open nextstep/Emacs.app —args -Q)

2. Create a buffer containing the following code and save it as bug.el:
(require 'ert)
(require 'testcover)

(ert-deftest should-error-bug ()
  (should-error (my-error)))

(defun my-error ()
  (/ 1 0))

3. M-x eval-buffer RET

4. M-x ert RET t RET

5. The test passes.

6. M-x testcover-start RET bug.el RET

7. M-x ert RET t RET

8. The test fails. TAB TAB d produced the attached debugger output.