But there will still be cases where the string is longer than 140 characers (including whitespaces) and the hastag shorter than 140 characters - I think this should be added as a test case.

Sorry , I don't know how to remove this, but it is wrong. I should look at the lenght of the hashtag and not the original string. There will be cases with the original string more than 140 charecters long that will give a hashtag less than 140 characters long.

I think it will be the other way around: There could be several instanses where len(s)>140 but your hashtag is not (after removing whitespaces). I made the same mistake (and did not get caught in the tests), but changed it afterwards.