tests: do not modify the existing app.json via jq

The jq binary doesn't support jsonc, so we can't modify it for testing purposes. Instead, just create an app.json with the correct heroku.postdeploy task.

tests/apps/python/app-heroku-postdeploy.json

@@ -0,0 +1,24 @@
+{
+  "healthchecks": {
+    "web": [
+      {
+        "attempts": 2,
+        "content": "python/http.server",
+        "name": "check-1",
+        "path": "/",
+        "timeout": 5,
+        "type": "startup",
+        "wait": 2
+      }
+    ]
+  },
+  "scripts": {
+    "dokku": {
+      // this is a comment for the postdeploy script
+      "postdeploy": "touch /app/postdeploy.test",
+      // this is a comment for the predeploy script
+      "predeploy": "touch /app/predeploy.test"
+    },
+    "postdeploy": "touch /app/heroku-postdeploy.test"
+  }
+}