SPDHG tests

[MargaretDuff]

Description

Removed lengthy tests comparing SPDHG and PDHG which relied on recalculated values and large tolerances and were expensive
Added two new SPDHG tests:

• Comparison with CVXPY
• Check the update rule for SPDHG does as expected, comparing against the algorithm in the original SPDHG paper

Related issues/links

Closes #2014

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

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