Add missing dependency on pytest-mock

For some reason, tox installs it on Jenkins, but not on my computer.

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Migrated from D5666 (view on Phabricator)

requirements-test.txt

@@ -2,6 +2,7 @@ pytest
 aioresponses
 pytest_asyncio
 pytest_flask
+pytest-mock
 swh.core[testing-core]
 swh.model[testing]
 asynctest