Fix mypy/click: add swh.core[testing] in requirements-test.txt

It now needs types-click which is indeed a dependency of
swh.core[testing].

requirements-test.txt

 pytest
 requests  # workaround for https://forge.softwareheritage.org/T2634
 requests-mock
+swh.core[testing]
 
 types-python-dateutil
 types-pyyaml