swh-vault's unit tests are broken on jenkins

with eg.

_______________ ERROR collecting swh/vault/tests/test_backend.py _______________
ImportError while importing test module '/home/jenkins/workspace/DVAU/tests/swh/vault/tests/test_backend.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
swh/vault/tests/test_backend.py:14: in <module>
    from swh.vault.tests.vault_testing import VaultTestFixture, hash_content
swh/vault/tests/vault_testing.py:12: in <module>
    from swh.storage.tests.storage_testing import StorageTestFixture
.tox/py3/lib/python3.5/site-packages/swh/storage/tests/__init__.py:9: in <module>
    from hypothesis.strategies import (binary, composite, sets)
E   ImportError: No module named 'hypothesis'

So the question is now: should hypothesis be in storage's requirements.txt instead of requirements-tests.txt?

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Migrated from T1354 (view on Phabricator)