tests.yml: add frank to dep installs

.github/workflows/tests.yml

@@ -17,6 +17,7 @@ jobs:
         # in __init__. below we'll reinstall for the tests.
         run: |
           pip install astropy
+          pip install frank
           pip install .
       - name: Cache/Restore the .mpol folder cache
         uses: actions/cache@v3