Update test.yml

Pin phpunit 9 to fix failing test with phpunit 10

Author: jfritschi

Diff for: .github/workflows/test.yml

@@ -15,13 +15,13 @@ jobs:
           - php: '7.2'
             phpunit: '8' 
           - php: '7.3'
-            phpunit: latest
+            phpunit: '9'
           - php: '7.4'
-            phpunit: latest
+            phpunit: '9'
           - php: '8.0'
-            phpunit: latest
+            phpunit: '9'
           - php: '8.1'
-            phpunit: latest
+            phpunit: '9'
     steps:
       - uses: actions/checkout@v2
       - uses: php-actions/composer@v6