fix(preview-service): only create a puppeteer client per app, not per request

[iainsproat]

Description & motivation

It looked wrong and I left a comment next to it that it looked wrong. Turns out it was wrong.

The puppeteer client should exist over the lifetime of the app. If it is created on each request, firstly that's expensive and time consuming to create but secondly it creates a conflict if another request is made while the first is still ongoing.

Changes:

• puppeteer client is created at app instantiation. It is passed in to the express server.
• in tests a stub client is created and passed in.

To-do before merge:

Screenshots:

Validation of changes:

Checklist:

• My pull request follows the guidelines in the Contributing guide?
• My pull request does not duplicate any other open Pull Requests for the same update/change?
• My commits are related to the pull request and do not amend unrelated code or documentation.
• My code follows a similar style to existing code.
• I have added appropriate tests.
• I have updated or added relevant documentation.

References