add mutation testing

infection.json5

@@ -0,0 +1,19 @@
+{
+    "$schema": "vendor/infection/infection/resources/schema.json",
+    "source": {
+        "directories": [
+            "src"
+        ]
+    },
+    "timeout": 30,
+    "logs": {
+        "text": "var/log/infection.txt",
+        "summary": "var/log/summary.log",
+    },
+    "mutators": {
+        "@default": true,
+        "global-ignore": [
+            "App\\Service\\Favicon::__toString"
+        ]
+    }
+}