Create initial Artifact 'abstractions' and preparations in testutils

Background

'Goals: Have abstracted anything within the tests which deals with the internals of an artifact into a TestArtifact class. This should not need additional tests, and should leave us with all tests still passing.'

Whilst adapting the current test api to 'Artifact', it's also important to examine and ensure that where possible specific tests internally aren't manually reliant on interacting with artifact internals and structure. (An instance of this was the local extract dir, which created paths based off artifact digest structure which could be asserted in a test, but has mostly been mitigated already)

This issue will also be used to highlight any areas in which we need to be accommodated for in the future implementation of AaaP.

Task description

• Ensure existing test API is handled via any necessary Artifact 'abstractions'
• Audit existing tests which interact with artifacts to ensure no localised dependencies on artifact structure
• MR for in-place changes, with notes on future accommodations for AaaP transition

Acceptance Criteria

'There exists a TestArtifact class in the testutils which allows tests to talk about the semantics of the internals of artifacts without actually caring about how they are represented on disk'. Tests behave as expected.

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