Move check for plus usage endpoint to test-with-plus target

[kate-osborn]

Proposed changes

Problem: Functional tests for OSS on forks are failing because the PLUS_USAGE_ENDPOINT isn't set.

Solution: Move check for PLUS_USAGE_ENDPOINT to the test-with-plus target.

Testing: testing in pipeline on fork

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

NONE

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.74%. Comparing base (895ce15) to head (2dc6bc4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2902   +/-   ##
=======================================
  Coverage   89.74%   89.74%           
=======================================
  Files         109      109           
  Lines       11150    11150           
  Branches       50       50           
=======================================
  Hits        10007    10007           
  Misses       1083     1083           
  Partials       60       60           

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