Correct autoloading for tests

Author: buffcode

composer.json

@@ -26,5 +26,10 @@
             "Prometheus\\": "src/"
         }
     },
+    "autoload-dev": {
+        "psr-0": {
+            "Test\\Prometheus\\": "tests/"
+        }
+    },
     "license": "Apache-2.0"
 }