Build/Test Tools: Allow the reusable PHPUnit workflow to test another repository.

A reusable workflow checks out the repository that calls it, so a caller that is not wordpress-develop cannot use this workflow and ends up maintaining a copy of it.

Add optional `repository` and `ref` inputs so the checkout can point elsewhere, and an `overlay-artifact` input that unpacks a same-run artifact over the checkout, for callers whose test files are not part of the repository being tested.

All three default to empty, so existing callers are unaffected: repository falls back to the calling repository, an empty ref is already checkout's own default and the overlay step is skipped.

Author: lucatume

.github/workflows/reusable-phpunit-tests-v3.yml

@@ -13,6 +13,16 @@ on:
         required: false
         type: 'string'
         default: 'ubuntu-24.04'
+      repository:
+        description: 'The repository to check out and test. Defaults to the repository calling this workflow.'
+        required: false
+        type: 'string'
+        default: ''
+      ref:
+        description: 'The branch, tag, or SHA to check out. Defaults to the ref that triggered the calling workflow.'
+        required: false
+        type: 'string'
+        default: ''
       php:
         description: 'The version of PHP to use, in the format of X.Y'
         required: true
@@ -82,6 +92,11 @@ on:
         required: false
         type: string
         default: ''
+      overlay-artifact:
+        description: 'The name of a same-workflow artifact whose contents are unpacked over the checkout. Optional: for callers whose test files are not part of the repository being tested.'
+        required: false
+        type: string
+        default: ''
     secrets:
       CODECOV_TOKEN:
         description: 'The Codecov token required for uploading reports.'
@@ -111,6 +126,7 @@ jobs:
   # Performs the following steps:
   # - Sets environment variables.
   # - Checks out the repository.
+  # - Unpacks the caller-provided overlay artifact over the checkout, if any.
   # - Downloads the prepared Gutenberg build provided by the calling workflow.
   # - Sets up Node.js.
   # - Sets up PHP.
@@ -143,9 +159,21 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
         with:
+          repository: ${{ inputs.repository || github.repository }}
+          ref: ${{ inputs.ref }}
           show-progress: ${{ runner.debug == '1' && 'true' || 'false' }}
           persist-credentials: false
 
+      # If the caller specifies an overlay artifact then unpack it over the checkout.
+      - name: Download overlay artifact
+        if: inputs.overlay-artifact != ''
+        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
+        with:
+          # Without a run ID, this action reads only from the caller's current workflow run.
+          name: ${{ inputs.overlay-artifact }}
+          path: .
+          digest-mismatch: error
+
       # Branches >= 5.9 call this workflow at @trunk without the producer job, so they
       # pass no artifact. They skip this step and fall back to a per-job download.
       - name: Download prepared Gutenberg build