Running the Firedrake test suite with different branches for subpackages

1. Create and check out a new Firedrake branch:

   $ cd firedrake/src/firedrake
   $ git checkout master
   $ git pull
   $ git checkout -b check-my-subpackage-changes
   

2. Modify the firedrake-install line in firedrake/.github/workflows/build.yml to include the argument --package-branch <subpackage> <branch> (e.g. --package-branch tsfc add-new-feature). Note that these branches must exist on the main package repository. Branches in forked repositories will not be found.
3. Commit and push this change.
4. Create a draft pull request on the Firedrake repository for this branch. Be sure to call the pull request something like "DO NOT MERGE Test new TSFC feature" or similar (example).