"wait-on" dependency hangs

[ChrisBAshton]

Describe the bug
We currently cannot run the test:ampValidate task as it relies on starting up a server and then running the AMP validator.

A dependency - wait-on - should handle listening for the server startup and then triggering the validator. But it currently hangs.

If we remove wait-on altogether, the validator is called too soon and the page can't be validated.

Arises from #1101.

To Reproduce
Steps to reproduce the behavior:

1. Run npm run test:ampValidate, which calls wait-on http://localhost:7080
2. Observe that the terminal hangs, and that amphtml-validator doesn't execure

Expected behavior
When the server has started, amphtml-validator should be triggered, and, pass or fail, the process should exit.

• Initially labelled with "bug"