Use single thread on parallel tests in CI

[icweaver]

Context: JuliaAstro/BackgroundMeshes.jl#19 (comment)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.70%. Comparing base (ce54078) to head (2b568a0).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #98   +/-   ##
=======================================
  Coverage   94.70%   94.70%           
=======================================
  Files          11       11           
  Lines         736      736           
=======================================
  Hits          697      697           
  Misses         39       39           

Flag	Coverage Δ
unittests	94.70% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.