Removed Projection Operator from SPDHG tests

[MargaretDuff]

Description

The Projection Operator in the SPDHG tests required astra to be installed. However, the projection operator was only being used to produce some example data, it wasn't actually used in the tests. I have removed this and am using some CIL example data instead.

Related issues/links

Closes #2025

Checklist

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• I have updated the relevant documentation
• I have implemented unit tests that cover any new or modified functionality
• CHANGELOG.md has been updated with any functionality change
• Request review from all relevant developers
• Change pull request label to 'Waiting for review'

Contribution Notes

Please read and adhere to the developer guide and local patterns and conventions.

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CIL (the Work) under the terms and conditions of the Apache-2.0 License
• I confirm that the contribution does not violate any intellectual property rights of third parties