Add PHPUnit tests for mu-plugins

.github/workflows/tests.yml

@@ -0,0 +1,29 @@
+name: CI
+
+on:
+  push:
+    branches: [ trunk, add/cdn-plugin, try/cdn-tests ]
+  pull_request:
+    branches: [ trunk ]
+
+  workflow_dispatch:
+
+jobs:
+  run-tests:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Install dependencies
+        run: |
+          npm install
+          composer install
+          npm run wp-env start
+          npm run build
+
+      - name: Run PHP unit tests
+        run: |
+          # Delete the default wp-env forced environment type
+          sed "/WP_ENVIRONMENT_TYPE/d" -i `npm run wp-env install-path 2>/dev/null | tail -n1`/tests-WordPress/phpunit-wp-config.php
+          npm run test:php

.wp-env.json

@@ -0,0 +1,15 @@
+{
+	"core": "WordPress/WordPress#master",
+	"mappings": {
+		"wp-content/mu-plugins": "./mu-plugins"
+	},
+	"env": {
+		"tests": {
+			"mappings": {
+				"vendor": "./vendor",
+				"phpunit": "./phpunit",
+				"phpunit.xml.dist": "./phpunit.xml.dist"
+			}
+		}
+	}
+}

composer.json

@@ -21,7 +21,10 @@
 		"composer/installers": "~1.0"
 	},
 	"require-dev": {
-		"wporg/wporg-repo-tools": "dev-trunk"
+		"phpunit/phpunit": "^9.5",
+		"wporg/wporg-repo-tools": "dev-trunk",
+		"wp-phpunit/wp-phpunit": "^5.9",
+		"yoast/phpunit-polyfills": "^1.0"
 	},
 	"config": {
 		"allow-plugins": {