chore: avoid running npm install for fixtures

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

We only need a subset of the modules available for tests, so stub them out instead.

[codecov]

Codecov Report

Merging #1009 into v14-develop will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           v14-develop   #1009   +/-   ##
===========================================
  Coverage          100%    100%           
===========================================
  Files               13      13           
  Lines              678     678           
===========================================
  Hits               678     678

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