Improve CI configuration

[swissspidy]

• Fix definition of global env variable
• Remove incorrect docker volumes
• Stop local environment after running e2e tests
• Add demo plugin that demonstrates how plugins can be used during e2e tests
• Add new CI step to run e2e tests as author

Related: #2795.

[westonruter]

Why is the demo plugin needed here?

[swissspidy]

@westonruter It basically acts as documentation to show how adding plugins for E2E tests works.

Alternatively we can just wait until we actually need some of these plugins when adding the first bunch of tests.

[westonruter]

I'm confused though: these tests are for the AMP plugin. Why would adding plugins need to be tested in this project?

[westonruter]

Unless the intention is to test that AMP validation errors are getting properly attributed?

[swissspidy]

@westonruter It's not about testing plugins. This change allows for adding some utility plugins that can be activated during E2E tests.

For example, such a plugin could create a custom post type and the accompanying E2E test would verify that block validation errors are shown for this custom post type too.