test_cost: use pytest.warns instead of raises for VisibleDeprecationWarning

[stephanlachnit]

Fixes the following warning:

=============================== warnings summary ===============================
tests/test_cost.py::test_UnbinnedNLL_visualize[False]
tests/test_cost.py::test_UnbinnedNLL_visualize[True]
  /tmp/autopkgtest-lxc.hry0h8si/downtmp/autopkgtest_tmp/tests/test_cost.py:244: VisibleDeprecationWarning: keyword 'nbins' is deprecated, please use 'bins'
    c.visualize((1, 2), nbins=20)

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
====== 597 passed, 2 skipped, 5 xfailed, 1 xpassed, 2 warnings in 17.25s =======

[HDembinski]

Good catch, thanks!