ci: improve test workflow configuration

- Prevent test workflow from running on tag pushes to avoid
  duplicate runs during releases
- Rename job from "build" to "test-matrix" for clarity

.github/workflows/test.yml

@@ -1,10 +1,15 @@
 # https://docs.djangoproject.com/en/stable/faq/install/#what-python-version-can-i-use-with-django
 name: test
 
-"on": [push, pull_request, workflow_dispatch]
+"on":
+  push:
+    branches:
+      - '**'
+  pull_request:
+  workflow_dispatch:
 
 jobs:
-  build:
+  test-matrix:
     runs-on: ubuntu-latest
     strategy:
       matrix: