Add a CI job to test against PHPUnit 10

.github/workflows/ci.yml

@@ -3,7 +3,7 @@ name: CI
 on:
   push:
   pull_request:
-  # Allow manually triggering the workflow.
+# Allow manually triggering the workflow.
   workflow_dispatch:
 
 # Cancels all previous workflow runs for the same branch that have not yet completed.
@@ -35,8 +35,13 @@ jobs:
       fail-fast: false
       matrix:
         php: ['7.3', '7.4', '8.0', '8.1']
+        include:
+          - name: with PHPUnit 10.1
+            php: '8.1'
+            phpunit: '10.1.x-dev' # change this with ^10.1 when released
+
 
-    name: PHP ${{ matrix.php }}
+    name: PHP ${{ matrix.php }} ${{ matrix.name }}
 
     steps:
       - name: Checkout code
@@ -49,6 +54,10 @@ jobs:
           ini-values: zend.assertions=1, error_reporting=-1, display_errors=On
           coverage: xdebug
 
+      - name: Lock to a specific PHPUnit version
+        if: matrix.phpunit
+        run: composer require --no-update --dev phpunit/phpunit:${{ matrix.phpunit }}
+
       # Install dependencies and handle caching in one go.
       # @link https://github.com/marketplace/actions/install-composer-dependencies
       - name: Install Composer dependencies