refactor(ci): drop dependency on PackSquash-action-test repository

This dependency is unnecessary, because that repository was focused on
testing the action, and to test the action we already have some
automated workflows at this repository.

For testing in other environments, the new reusable workflow step
commands can be directly copied and pasted to an Unix-like shell, which
is almost as easy as running `git clone`, but with the added advantage
that at least someone (me) will look at and update this commands
periodically.

While at it, let's update the referenced pack versions to test more
up-to-date packs, which are likely to be more representative of end
PackSquash usage.

.github/workflows/actions/set_up_repo_with_submodules/action.yml

@@ -0,0 +1,36 @@
+name: Set up empty Git repository with public pack submodules
+description: Reusable steps to set up a repository with packs in submodules for testing
+runs:
+  using: composite
+  steps:
+    # The packs checked out by these steps are distributed to the public
+    # and, to the best of our knowledge, used in conformance with the
+    # usage rights given in their licenses, which are, in no particular
+    # order:
+    # - CC BY 4.0,
+    # - CC BY-NC-SA 4.0,
+    # - Custom licenses.
+    # Thanks to their authors for making such great packs!
+    - name: Set up empty Git repository with public pack submodules
+      shell: sh
+      run: |
+        git -c init.defaultBranch=master init
+
+        while read -r submodule_info; do
+          url="${submodule_info%% *}"
+          name="${submodule_info#* }"
+          name="${name% *}"
+          commitish="${submodule_info##* }"
+
+          git submodule add "$url" "$name"
+          git -C "$name" -c advice.detachedHead=false checkout "$commitish"
+          git add "$name"
+        done <<'SUBMODULES'
+        https://github.com/PoeticRainbow/golden-days.git golden-days 1.9.0
+        https://github.com/Crackers0106/Programmer-Art-Plus.git Programmer-Art-Plus 8ddc173e274094588f04d1c989a58378fb56b303
+        https://github.com/SummerFields/SummerFields.git SummerFields 645be0fc27495117a7decc96fee810ff1ed584ab
+        SUBMODULES
+
+        GIT_AUTHOR_NAME='github-actions[bot]' GIT_AUTHOR_EMAIL='41898282+github-actions[bot]@users.noreply.github.com' \
+        GIT_COMMITTER_NAME="$GIT_AUTHOR_NAME" GIT_COMMITTER_EMAIL="$GIT_AUTHOR_EMAIL" \
+        git commit -m 'Root commit'

.github/workflows/test_pack_in_submodule.yml

@@ -13,27 +13,19 @@ jobs:
     if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.fork
 
     steps:
-      - name: Clone pack repository
-        uses: actions/checkout@v3
-        with:
-          repository: ComunidadAylas/PackSquash-action-test
-          fetch-depth: 0
-          submodules: recursive
-          token: ${{ secrets.GITHUB_TOKEN }}
       - name: Clone action repository
         uses: actions/checkout@v3
         with:
-          path: action-checkout
-      - name: Modify pack files
+          path: action
+      - name: Set up empty Git repository with public pack submodules
+        uses: ./action/.github/workflows/actions/set_up_repo_with_submodules
+      - name: Modify pack files (test that working directory changes are handled well)
         run: |
-          echo '{ "pack": { "pack_format": 8, "description": "Dummy pack.mcmeta" } }' > golden-days/golden-days-base/pack.mcmeta
           touch golden-days/golden-days-base/assets/minecraft/sounds.json
           mv golden-days/golden-days-base/credits.txt golden-days/golden-days-base/credits_new.txt
-          # Remove some files that generate GStreamer negotiation errors on v0.3.1.
-          # This also serves to check that the action handles files being deleted from the working copy correctly
-          rm golden-days/golden-days-base/assets/minecraft/sounds/liquid/*.ogg
+          rm golden-days/golden-days-base/assets/minecraft/optifine/color.properties
       - name: Run PackSquash
-        uses: ./action-checkout
+        uses: ./action
         with:
           packsquash_version: latest
           options: |

.github/workflows/test_pack_submodule.yml

@@ -13,19 +13,14 @@ jobs:
     if: github.event_name != 'pull_request' || github.event.pull_request.head.repo.fork
 
     steps:
-      - name: Clone pack repository
-        uses: actions/checkout@v3
-        with:
-          repository: ComunidadAylas/PackSquash-action-test
-          fetch-depth: 0
-          submodules: recursive
-          token: ${{ secrets.GITHUB_TOKEN }}
       - name: Clone action repository
         uses: actions/checkout@v3
         with:
-          path: action-checkout
+          path: action
+      - name: Set up empty Git repository with public pack submodules
+        uses: ./action/.github/workflows/actions/set_up_repo_with_submodules
       - name: Run PackSquash
-        uses: ./action-checkout
+        uses: ./action
         with:
           packsquash_version: latest
           options: |