Fix missing coverage files in GitHub Actions

As of September 2nd, using the `upload-artifact` GitHub Action no longer uploads hidden files by default:

https://github.blog/changelog/2024-08-19-notice-of-upcoming-deprecations-and-breaking-changes-in-github-actions-runners/

This breaks uploading of `.coverage.*` files used to combine multiple coverage reports.

The `upload-artifact` README now includes this guidance on including hidden files: https://github.com/actions/upload-artifact?tab=readme-ov-file#uploading-hidden-files

.github/workflows/test.yml

@@ -62,6 +62,7 @@ jobs:
         with:
           name: coverage-${{ matrix.python }}-${{ matrix.django }}-${{ matrix.wagtail }}
           path: .coverage.*
+          include-hidden-files: true
 
   coverage:
     name: coverage