Test suite: code linting

[deining]

This PR brings the work that started with #5364 to an end.

Checklist:

• Tests all pass.

[coveralls]

coverage: 59.306% (+0.01%) from 59.294%
when pulling 9dd42c3 on deining:test-linting
into d47fdda on fyne-io:develop.

[andydotxyz]

Sorry there has been a lot of change to the testing infrastructure with the thread changes landing this week.
Could you bring in latest develop and make sure you're happy with the results? :) Thanks

[deining]

Sorry there has been a lot of change to the testing infrastructure with the thread changes landing this week. Could you bring in latest develop and make sure you're happy with the results? :) Thanks

I rebased my PR and made the necessary adjustments. All checkers are green now, except for static-analysis, which is addressed in PR #5379.