tests/avocado: Make fetch_asset() unconditionally require a crypto hash

tests/avocado: Make fetch_asset() unconditionally require a crypto hash

In a perfect world we'd have reproducible tests,
but then we'd be sure we run the same binaries.
If a binary artifact isn't hashed, we have no idea
what we are running. Therefore enforce hashing for
all our artifacts.

With this change, unhashed artifacts produce:

  $ avocado run tests/avocado/multiprocess.py
   (1/2) tests/avocado/multiprocess.py:Multiprocess.test_multiprocess_x86_64:
   ERROR: QemuBaseTest.fetch_asset() missing 1 required positional argument: 'asset_hash' (0.19 s)

Inspired-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-ID: <20231115205149.90765-1-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>