tests: Use PSR-4-style namespaces (#2778)

We cannot yet switch to namespaces for RSS-Bridge itself but for tests we are not limited by BC.
It does not actually do anything since PHPUnit will search for the test files without the help of the autoloader but it still makes the directory cleaner.

composer.json

@@ -39,6 +39,11 @@
         "ext-sqlite3": "Allows to use an SQLite database for caching",
         "ext-dom": "Allows to use some bridges based on XPath expressions"
     },
+    "autoload-dev": {
+        "psr-4": {
+            "RssBridge\\Tests\\": "tests"
+        }
+    },
     "scripts": {
         "test": "./vendor/bin/phpunit",
         "lint": "./vendor/bin/phpcs --standard=phpcs.xml --warning-severity=0 --extensions=php -p ./",