requirements-test: Add missing swh.loader.core[testing] dependency

Side effect of swh.loader.core v5.18.0 release.

requirements-test.txt

 pytest >= 8.1
 pytest-mock
 requests_mock
+swh.loader.core[testing]
 swh.scheduler[testing] >= 0.5.0
 swh.storage[testing]
 types-Deprecated