Set random seed for tests

[tsmbland]

Description

We sometimes find that tests randomly fail due to issues with randomly generated test data. Ideally we'd design the tests/fixtures in a way that wasn't so sensitive to this, but for now an easy solution is to set the random seed to a value that we know produces feasible test data.

(I initially tried 0 as the seed and all unit test runs failed, then switched to 123 and they all passed)

Fixes # (issue)

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (non-breaking, back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (whatever its nature)

Key checklist

• All tests pass: $ python -m pytest
• The documentation builds and looks OK: $ python -m sphinx -b html docs docs/build

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[dalonsoa]

Looks good to me. Theory says that a good seed for a RNG is a large, prime number, but I don't think that's super relevant in this case.