Add 402_STABLE GHA tests

Author: stronk7

.github/workflows/test.yml

@@ -77,6 +77,8 @@ jobs:
         include:
           - php: '8.1'
             moodle-branch: 'master'
+          - php: '8.1'
+            moodle-branch: 'MOODLE_402_STABLE'
           - php: '8.1'
             moodle-branch: 'MOODLE_401_STABLE'
           - php: '8.0'