fix(tests): adapt app.json fixtures and tini test for env processing

Pre-set LOCAL_DOMAIN in the tini test so mastodon's required env var
resolves, and replace unsupported `generator: "echo 5"` with `value: "5"`
in the nodejs-express and dockerfile fixtures so WEB_CONCURRENCY no
longer aborts deploys via the new app.json env processor.

tests/apps/nodejs-express/app.json

@@ -16,7 +16,7 @@
     },
     "WEB_CONCURRENCY": {
       "description": "The number of processes to run.",
-      "generator": "echo 5"
+      "value": "5"
     }
   },
   "healthchecks": {