Ensure support for external nodes in unit tests

[QMalcolm]

resolves #8944

Problem

We weren't sure if external nodes in unit tests would just work, and it turns out that they do!

Note: this wasn't originally working

Before the winter break this didn't just work. I started on this work early in December, and at that time it was failing to find the external node at ref resolution time. This was because under the hood the identifier for refs in unit tests are under a different name to spoof the node to be ref'd. At the time there was a disconnect between the ref in the unit test, and the unit test in the model.

Solution

Added a test which includes an external node

Note

There is no changelog for this PR because nothing in core itself was changed, just an added test.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (56dfb34) 86.88% compared to head (bd7dfda) 85.96%.

Additional details and impacted files

@@                       Coverage Diff                       @@
##           unit_testing_feature_branch    #9271      +/-   ##
===============================================================
- Coverage                        86.88%   85.96%   -0.92%     
===============================================================
  Files                              223      223              
  Lines                            27657    27657              
===============================================================
- Hits                             24029    23775     -254     
- Misses                            3628     3882     +254     

Flag	Coverage Δ
integration	82.77% <ø> (-1.13%)	⬇️
unit	64.87% <ø> (ø)

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[QMalcolm]

I was wrong this doesn't just work. I just forgot that make test doesn't run the integration tests locally, I have to run make integration 🙃 Doing so results in the same failure I was receiving prior to winter break, which means the recent updates the feature branch haven't resolved the issue.