Add local test server for E2E tests

[sidvishnoi]

Presently, we rely on https://webmonetization.org/play/ to add link tags on a page for automated testing. This has some issues:

• Playground includes lots of other things which add up as noise in testing (clicking button to add link etc. although the ?wa URL params were helpful)
• Playground can only dynamically add link tags with JS, in a single manner.
• We can't have custom CSP or other headers there
• Even if small, there's a performance overhead on using a remote playground website.

We want to set up a local server of test pages that we can use in the extension.
The test pages will be version controlled along with the extension, and also serve as test pages for other implementations and users.

In future, we might want to run the test server on additional domains (for CSP/Permissions Policy specific testing, instead of just localhost:{port}.

Aside: #1027 enabled us to run the extension on localhost.