Fix F541: replace f-strings with no placeholders in lightning tests

[nikunjkumar05]

About PR

This PR fixes flake8 F541 (f-strings missing placeholders) by replacing f-strings that contain no placeholders with plain strings in the lightning test files. These were lint errors only but reduce noise and avoid confusion.

Why

F541 indicates an f-string with no {} placeholders — likely a typo. Replacing with a plain string removes the lint error and clarifies intent.

How to Test :

flake8 lightning/tests || true

Checklist

Before submitting the PR, ensure the following tasks are completed. If an item is not applicable to your PR, please mark it as checked:

• [✓] The changelog has been updated in the relevant commit(s) according to the guidelines.
• [✓] Tests have been added or modified to reflect the changes.
• [✓] Documentation has been reviewed and updated as needed.
• [✓] Related issues have been listed and linked, including any that this PR closes.
• [✓] Important All PRs must consider how to reverse any persistent changes for tools/lightning-downgrade