Add linting in tests and demos folder

[bidoubiwa]

as pr #134 a linter is introduced but only in the source code. For consistency Tests and Demos folder should be lint-ed as well.

[ppamorim]

I will have to disable the lint Explicit Type Interface Violation because it's giving a false positive on Swift 5.4, here. Is that ok?

[bidoubiwa]

Ah that's a shame! Yes lets disable it. Could you make a comment next to the rule in the config file explaining the false positive and maybe also a small issue explaining the issue so that we can track future updates of swiflint and update it when it is fixed?

If you prefer not I can do the PR and the issue no problem :) Thanks for investigating !