chore: add test for NODE_ENV in server process for #184

[bahmutov]

• closes How to set NODE_ENV=test? #184

Adds an end-to-end test that checks if the server has the right NODE_ENV environment variable when we run it

To verify manually run in test/test-node-env folder

$ NODE_ENV=foo npm run demo

> test-node-env@ demo /Users/gleb/git/start-server-and-test/test/test-node-env
> node ../../src/bin/start.js 9000

starting server using command "npm run start"
and when url "http://localhost:9000" is responding
running tests using command "npm run test"

> test-node-env@ start /Users/gleb/git/start-server-and-test/test/test-node-env
> node ../server

sleeping for 5 seconds before starting
listening at port 9000
HEAD
HEAD
HEAD

> test-node-env@ test /Users/gleb/git/start-server-and-test/test/test-node-env
> echo server has NODE_ENV=$NODE_ENV

server has NODE_ENV=foo