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

[nikunjkumar05]

Changelog-None

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

• Changelog-None
• 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

[nikunjkumar05]

@rustyrussell @daywalker90 @Lagrang3 please review my PR
thank you

[daywalker90]

You need to add "Changelog-None" to the PR or commit message. Otherwise looks good to me.

[nikunjkumar05]

ok sir

[nikunjkumar05]

@rustyrussell @daywalker90 @Lagrang3
Thank you for review
changes addressed sir please review the PR

[madelinevibes]

@nikunjkumar05 thanks for your contribution. You haven't replaced all of the possible F541 (f-strings missing placeholders). There are a lot of different files that have this dev string. We will holding off reviewing further until the PR has all of the replacements required.

[nikunjkumar05]

Ok @madelinevibes thank you for review I will fix it shortly