Fix Github Actions setup to install Django after Wagtail

This way, if the Django version is outside Wagtail's supported range (e.g. we're testing against Django's git main), it won't get silently downgraded.

.github/workflows/test.yml

@@ -24,7 +24,7 @@ on:
 # - django 3.2, python 3.10, sqlite
 # - django 3.2, python 3.9, postgres, USE_EMAIL_USER_MODEL=yes
 # - django 3.2, python 3.10, postgres, DISABLE_TIMEZONE=yes
-# - django stable/3.2.x, python 3.9, postgres (allow failures)
+# - django stable/4.0.x, python 3.9, postgres (allow failures)
 # - django main, python 3.10, postgres (allow failures)
 # - elasticsearch 5, django 3.0, python 3.7, sqlite
 # - elasticsearch 6, django 3.1, python 3.7, postgres
@@ -51,8 +51,8 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install "${{ matrix.django }}"
           pip install -e .[testing]
+          pip install "${{ matrix.django }}"
       - name: Test
         run: |
           ./runtests.py
@@ -103,8 +103,8 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install "psycopg2>=2.6"
-          pip install "${{ matrix.django }}"
           pip install -e .[testing]
+          pip install "${{ matrix.django }}"
       - name: Test
         run: |
           ./runtests.py
@@ -144,8 +144,8 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install "mysqlclient>=1.4,<2"
-          pip install "${{ matrix.django }}"
           pip install -e .[testing]
+          pip install "${{ matrix.django }}"
       - name: Test
         run: |
           ./runtests.py
@@ -186,8 +186,8 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install "${{ matrix.django }}"
           pip install -e .[testing]
+          pip install "${{ matrix.django }}"
           pip install "elasticsearch>=5,<6"
           pip install certifi
       - name: Test
@@ -227,8 +227,8 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install "${{ matrix.django }}"
           pip install -e .[testing]
+          pip install "${{ matrix.django }}"
           pip install "elasticsearch>=7,<8"
           pip install certifi
       - name: Test
@@ -274,8 +274,8 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install "psycopg2>=2.6"
-          pip install "${{ matrix.django }}"
           pip install -e .[testing]
+          pip install "${{ matrix.django }}"
           pip install "elasticsearch>=6,<7"
           pip install certifi
       - name: Test
@@ -322,8 +322,8 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install "psycopg2>=2.6"
-          pip install "${{ matrix.django }}"
           pip install -e .[testing]
+          pip install "${{ matrix.django }}"
           pip install "elasticsearch>=7,<8"
           pip install certifi
       - name: Test