Fulfil Test Suite for skeleton version

[BelowBayesline]

At some point we need to re-approach tests.

We should implement something like per-module test files, with test functions for each key functionality and minor functionality in that module.

For example we could create test_pipeline.py which contains all the tests for pipeline.py, and so on for each module.

This will be a little arduous, and we should really sketch this out in a series of meetings about what is our testing policy/approach and how do we implement it. Not difficult, just a bit of a time sink!

This isn't the only approach but it is a logical one to me. Open to any other testing approach suggestions as we work/discuss!

[pikes-ons]

Reviewed tests by module:
stage.py - complete
errors.py - no tests needed as only holds error message categories
execution.py - in progress
graph.py - to do
loader.py - to do
logger.py - to do
models.py - in progress
pipeline.py - to do after Configuration methods set up
run_pipeline.py - tests not needed
runner.py - to do, mostly covered by integration tests and running the example pipelines as a test however other methods may need tests.
warnings.py - no tests needed as only holds warning message categories

Consider whether we want copilot to review our tests to ensure that every avenue is covered once we've completed them however this would lead to very detailed tests and unsure how 'complete' we want to be.

Sub-issues created for each module

[pikes-ons]

Moved to backlog due to pause on testing development until after major architecture changes are complete