Initialize invocation context before test fixture creation.

[peterallenwebb]

Problem

The code used to set up test fixtures sometimes uses environment variables via the new InvocationContext mechanism, but it was not always initialized in time.

Solution

Add an initialization function to the test fixtures, and set an invocation context in that function.

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 (15dcb9a) 87.93% compared to head (9d4c62e) 87.89%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9529      +/-   ##
==========================================
- Coverage   87.93%   87.89%   -0.04%     
==========================================
  Files         167      167              
  Lines       22172    22176       +4     
==========================================
- Hits        19497    19492       -5     
- Misses       2675     2684       +9     

Flag	Coverage Δ
integration	85.47% <100.00%> (-0.11%)	⬇️
unit	61.84% <60.00%> (+<0.01%)	⬆️

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[QMalcolm]

Thank you so much for hunting down this issue 🙇