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

@@ -2,6 +2,7 @@ attrs
 dulwich >= 0.18.7
 pytest
 pytest-mock
+swh.core[testing]
 swh.loader.core
 swh.loader.git >= 0.8
 swh.storage[testing]