Validate that perturbed answer files are accepted by output validators

[simonlindholm]

We just had a case in http://pokval19.kattis.com/ where output that doesn't end in a newline didn't get accepted by an output validator. It would be good if verifyproblem checked this, similar to how it checks the junk output is rejected, and that input validators aren't too liberal. We should check that outputs are still accepted when:

• additional whitespace is introduced (e.g. replace every \s by two spaces, add spaces to beginning/end)
• whitespace is removed, at the end or otherwise
• capitalization is changed
• numbers are formatted differently, e.g. with leading zeroes

There needs to be some way of opting out of this; reusing the space_change_sensitive and case_sensitive flags might be reasonable?