Specify branches for test CI runs

[dstansby]

This should prevent duplicate CI, e.g., as in #775

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.92%. Comparing base (506c89b) to head (889f9dc).
⚠️ Report is 8 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (506c89b) and HEAD (889f9dc). Click for more details.

HEAD has 10 uploads less than BASE

Flag	BASE (506c89b)	HEAD (889f9dc)
	15	5

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #776      +/-   ##
==========================================
- Coverage   99.96%   90.92%   -9.04%     
==========================================
  Files          64       64              
  Lines        2789     2789              
==========================================
- Hits         2788     2536     -252     
- Misses          1      253     +252     

see 2 files with indirect coverage changes

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