Fix filename of CVE test (ipython#15099)

From a local pytest run, and also the most recent CI run on main
(https://github.com/ipython/ipython/actions/runs/19927362702/job/57130406121)
it looks like `tests/cve.py` is not executed at all because its file
name does not start with `test_`. This PR fixes that

(the test does still pass, thankfully)

Author: Carreau