Add testing exports documentation to API docs

[theluckystrike]

Adds documentation for the testing exports (injectionStrings, Failscape, Stubscape, Throwscape) to the API documentation. These exports from shescape/testing were not documented in the API docs.

[ericcornelissen]

Thanks for taking the time to contribute!

This API is covered in testing.md, if you think there's anything missing please add it there (and revert the changes to api.md). If not, please close this Pull Request.

Also, off-topic, but #2396 was about testing.js (not .md) - which is now already covered by #2399.

[theluckystrike]

Thank you for the clarification! You're right — the testing exports are already well-documented in testing.md. I'll close this PR since it's not needed.

Apologies for the noise, and thanks for maintaining shescape!