in headless tests on example app we can use npm wait-port instead of sleep 5

[vincanger]

https://github.com/wasp-lang/wasp/blob/main/waspc/headless-test/package.json#L10

here is a relevant discussion: https://github.com/wasp-lang/open-saas/pull/69#discussion_r1515102649

[infomiho]

I believe I waited the extra 5 seconds to get less flaky behaviour with Playwright waiting for the web app to start properly. It might not be entirely necessary, but the change needs to be tested.