Updates the documentation for running the integration tests

[mhkuu]

Summary

This PR updates the documentation for running the integration tests.

Relevant technical choices

Technical choices that affect more than this issue:
None.

Test instructions

This PR can be acceptance tested by following these steps:

1. Open the instructions on development/environment/setup-plugin-integration-tests/.
2. Confirm the instructions make more sense than they used to.

Quality assurance

• Security - I have thought about any security implications this code might add.
• Performance - I have checked that this code doesn't impact performance (greatly).
• Caching - I have analyzed the caching methods that this code touches and have added instructions to deal with those.
• Tested - I have tested this code to the best of my abilities.
• Automated tests - I have added unit tests to verify the code works as intended.
• Testability - I have added unique ids to elements, so they can be located in automated testing.
• I have altered a filename.
  • I have adjusted the ID property accordingly and updated all internal links.
  • I have added the redirect to the _redirects file in the root of the project.

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