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/unit/test_helper.bash

@@ -561,15 +561,6 @@ add_release_command() {
   echo "release: touch /app/release.test" >>"$APP_REPO_DIR/Procfile"
 }
 
-add_postdeploy_command() {
-  local APP="$1"
-  local APP_REPO_DIR="$2"
-  [[ -z "$APP" ]] && local APP="$TEST_APP"
-  touch "$APP_REPO_DIR/app.json"
-  contents=$(jq '.scripts.postdeploy = "touch /app/heroku-postdeploy.test"' "$APP_REPO_DIR/app.json")
-  echo "${contents}" >"$APP_REPO_DIR/app.json"
-}
-
 move_dockerfile_into_place() {
   local APP="$1"
   local APP_REPO_DIR="$2"